fix: address PR #905 review — migration error handling, backfill heuristic, test comment

1. Migration ALTER error no longer swallowed: check error from ALTER TABLE
   and return if it fails (unless column already exists). Migration is not
   marked complete on failure.

2. Backfill heuristic fixed: use observations table JOIN instead of
   packet_count > 0, since UpsertObserver sets packet_count = 1 on INSERT
   even for status-only observers.

3. Test clarifying comment: document that InsertTransmission uses
   data.Timestamp (not time.Now()) as source-of-truth for last_packet_at,
   so the hardcoded assertion is correct.

cmd/ingestor/db.go

@@ -113,7 +113,8 @@ func applySchema(db *sql.DB) error {
 			battery_mv INTEGER,
 			uptime_secs INTEGER,
 			noise_floor REAL,
-			inactive INTEGER DEFAULT 0
+			inactive INTEGER DEFAULT 0,
+			last_packet_at TEXT DEFAULT NULL
 		);
 
 		CREATE INDEX IF NOT EXISTS idx_nodes_last_seen ON nodes(last_seen);
@@ -418,6 +419,28 @@ func applySchema(db *sql.DB) error {
 		log.Println("[migration] observations.raw_hex column added")
 	}
 
+	// Migration: add last_packet_at column to observers (#last-packet-at)
+	row = db.QueryRow("SELECT 1 FROM _migrations WHERE name = 'observers_last_packet_at_v1'")
+	if row.Scan(&migDone) != nil {
+		log.Println("[migration] Adding last_packet_at column to observers...")
+		_, alterErr := db.Exec(`ALTER TABLE observers ADD COLUMN last_packet_at TEXT DEFAULT NULL`)
+		if alterErr != nil && !strings.Contains(alterErr.Error(), "duplicate column") {
+			return fmt.Errorf("observers last_packet_at ALTER: %w", alterErr)
+		}
+		// Backfill: set last_packet_at = last_seen only for observers that actually have
+		// observation rows (packet_count alone is unreliable — UpsertObserver sets it to 1
+		// on INSERT even for status-only observers).
+		res, err := db.Exec(`UPDATE observers SET last_packet_at = last_seen
+			WHERE last_packet_at IS NULL
+			AND rowid IN (SELECT DISTINCT observer_idx FROM observations WHERE observer_idx IS NOT NULL)`)
+		if err == nil {
+			n, _ := res.RowsAffected()
+			log.Printf("[migration] Backfilled last_packet_at for %d observers with packets", n)
+		}
+		db.Exec(`INSERT INTO _migrations (name) VALUES ('observers_last_packet_at_v1')`)
+		log.Println("[migration] observers.last_packet_at column added")
+	}
+
 	return nil
 }
 
@@ -501,7 +524,7 @@ func (s *Store) prepareStatements() error {
 		return err
 	}
 
-	s.stmtUpdateObserverLastSeen, err = s.db.Prepare("UPDATE observers SET last_seen = ? WHERE rowid = ?")
+	s.stmtUpdateObserverLastSeen, err = s.db.Prepare("UPDATE observers SET last_seen = ?, last_packet_at = ? WHERE rowid = ?")
 	if err != nil {
 		return err
 	}
@@ -580,9 +603,9 @@ func (s *Store) InsertTransmission(data *PacketData) (bool, error) {
 		err := s.stmtGetObserverRowid.QueryRow(data.ObserverID).Scan(&rowid)
 		if err == nil {
 			observerIdx = &rowid
-			// Update observer last_seen on every packet to prevent
+			// Update observer last_seen and last_packet_at on every packet to prevent
 			// low-traffic observers from appearing offline (#463)
-			_, _ = s.stmtUpdateObserverLastSeen.Exec(now, rowid)
+			_, _ = s.stmtUpdateObserverLastSeen.Exec(now, now, rowid)
 		}
 	}
 

cmd/ingestor/db_test.go

@@ -569,6 +569,61 @@ func TestInsertTransmissionUpdatesObserverLastSeen(t *testing.T) {
 	}
 }
 
+func TestLastPacketAtUpdatedOnPacketOnly(t *testing.T) {
+	s, err := OpenStore(tempDBPath(t))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer s.Close()
+
+	// Insert observer via status path — last_packet_at should be NULL
+	if err := s.UpsertObserver("obs1", "Observer1", "SJC", nil); err != nil {
+		t.Fatal(err)
+	}
+
+	var lastPacketAt sql.NullString
+	s.db.QueryRow("SELECT last_packet_at FROM observers WHERE id = ?", "obs1").Scan(&lastPacketAt)
+	if lastPacketAt.Valid {
+		t.Fatalf("expected last_packet_at to be NULL after UpsertObserver, got %s", lastPacketAt.String)
+	}
+
+	// Insert a packet from this observer — last_packet_at should be set
+	data := &PacketData{
+		RawHex:      "0A00D69F",
+		Timestamp:   "2026-04-24T12:00:00Z",
+		ObserverID:  "obs1",
+		Hash:        "lastpackettest123456",
+		RouteType:   2,
+		PayloadType: 2,
+		PathJSON:    "[]",
+		DecodedJSON: `{"type":"TXT_MSG"}`,
+	}
+	if _, err := s.InsertTransmission(data); err != nil {
+		t.Fatal(err)
+	}
+
+	s.db.QueryRow("SELECT last_packet_at FROM observers WHERE id = ?", "obs1").Scan(&lastPacketAt)
+	if !lastPacketAt.Valid {
+		t.Fatal("expected last_packet_at to be non-NULL after InsertTransmission")
+	}
+	// InsertTransmission uses `now = data.Timestamp || time.Now()`, so last_packet_at
+	// should match the packet's Timestamp when provided (same source-of-truth as last_seen).
+	if lastPacketAt.String != "2026-04-24T12:00:00Z" {
+		t.Errorf("expected last_packet_at=2026-04-24T12:00:00Z, got %s", lastPacketAt.String)
+	}
+
+	// UpsertObserver again (status path) — last_packet_at should NOT change
+	if err := s.UpsertObserver("obs1", "Observer1", "SJC", nil); err != nil {
+		t.Fatal(err)
+	}
+
+	var lastPacketAtAfterStatus sql.NullString
+	s.db.QueryRow("SELECT last_packet_at FROM observers WHERE id = ?", "obs1").Scan(&lastPacketAtAfterStatus)
+	if !lastPacketAtAfterStatus.Valid || lastPacketAtAfterStatus.String != lastPacketAt.String {
+		t.Errorf("UpsertObserver should not change last_packet_at; expected %s, got %v", lastPacketAt.String, lastPacketAtAfterStatus)
+	}
+}
+
 func TestEndToEndIngest(t *testing.T) {
 	s, err := OpenStore(tempDBPath(t))
 	if err != nil {

cmd/server/db.go

@@ -170,6 +170,7 @@ type Observer struct {
 	BatteryMv     *int     `json:"battery_mv"`
 	UptimeSecs    *int64   `json:"uptime_secs"`
 	NoiseFloor    *float64 `json:"noise_floor"`
+	LastPacketAt  *string  `json:"last_packet_at"`
 }
 
 // Transmission represents a row from the transmissions table.
@@ -972,7 +973,7 @@ func (db *DB) getObservationsForTransmissions(txIDs []int) map[int][]map[string]
 
 // GetObservers returns all observers sorted by last_seen DESC.
 func (db *DB) GetObservers() ([]Observer, error) {
-	rows, err := db.conn.Query("SELECT id, name, iata, last_seen, first_seen, packet_count, model, firmware, client_version, radio, battery_mv, uptime_secs, noise_floor FROM observers ORDER BY last_seen DESC")
+	rows, err := db.conn.Query("SELECT id, name, iata, last_seen, first_seen, packet_count, model, firmware, client_version, radio, battery_mv, uptime_secs, noise_floor, last_packet_at FROM observers ORDER BY last_seen DESC")
 	if err != nil {
 		return nil, err
 	}
@@ -983,7 +984,7 @@ func (db *DB) GetObservers() ([]Observer, error) {
 		var o Observer
 		var batteryMv, uptimeSecs sql.NullInt64
 		var noiseFloor sql.NullFloat64
-		if err := rows.Scan(&o.ID, &o.Name, &o.IATA, &o.LastSeen, &o.FirstSeen, &o.PacketCount, &o.Model, &o.Firmware, &o.ClientVersion, &o.Radio, &batteryMv, &uptimeSecs, &noiseFloor); err != nil {
+		if err := rows.Scan(&o.ID, &o.Name, &o.IATA, &o.LastSeen, &o.FirstSeen, &o.PacketCount, &o.Model, &o.Firmware, &o.ClientVersion, &o.Radio, &batteryMv, &uptimeSecs, &noiseFloor, &o.LastPacketAt); err != nil {
 			continue
 		}
 		if batteryMv.Valid {
@@ -1006,8 +1007,8 @@ func (db *DB) GetObserverByID(id string) (*Observer, error) {
 	var o Observer
 	var batteryMv, uptimeSecs sql.NullInt64
 	var noiseFloor sql.NullFloat64
-	err := db.conn.QueryRow("SELECT id, name, iata, last_seen, first_seen, packet_count, model, firmware, client_version, radio, battery_mv, uptime_secs, noise_floor FROM observers WHERE id = ?", id).
-		Scan(&o.ID, &o.Name, &o.IATA, &o.LastSeen, &o.FirstSeen, &o.PacketCount, &o.Model, &o.Firmware, &o.ClientVersion, &o.Radio, &batteryMv, &uptimeSecs, &noiseFloor)
+	err := db.conn.QueryRow("SELECT id, name, iata, last_seen, first_seen, packet_count, model, firmware, client_version, radio, battery_mv, uptime_secs, noise_floor, last_packet_at FROM observers WHERE id = ?", id).
+		Scan(&o.ID, &o.Name, &o.IATA, &o.LastSeen, &o.FirstSeen, &o.PacketCount, &o.Model, &o.Firmware, &o.ClientVersion, &o.Radio, &batteryMv, &uptimeSecs, &noiseFloor, &o.LastPacketAt)
 	if err != nil {
 		return nil, err
 	}

cmd/server/db_test.go

@@ -48,7 +48,8 @@ func setupTestDB(t *testing.T) *DB {
 			radio TEXT,
 			battery_mv INTEGER,
 			uptime_secs INTEGER,
-			noise_floor REAL
+			noise_floor REAL,
+			last_packet_at TEXT DEFAULT NULL
 		);
 
 		CREATE TABLE transmissions (
@@ -355,6 +356,10 @@ func TestGetObservers(t *testing.T) {
 	if observers[0].ID != "obs1" {
 		t.Errorf("expected obs1 first (most recent), got %s", observers[0].ID)
 	}
+	// last_packet_at should be nil since seedTestData doesn't set it
+	if observers[0].LastPacketAt != nil {
+		t.Errorf("expected nil LastPacketAt for obs1 from seed, got %v", *observers[0].LastPacketAt)
+	}
 }
 
 func TestGetObserverByID(t *testing.T) {
@@ -369,6 +374,48 @@ func TestGetObserverByID(t *testing.T) {
 	if obs.ID != "obs1" {
 		t.Errorf("expected obs1, got %s", obs.ID)
 	}
+	// Verify last_packet_at is nil by default
+	if obs.LastPacketAt != nil {
+		t.Errorf("expected nil LastPacketAt, got %v", *obs.LastPacketAt)
+	}
+}
+
+func TestGetObserverLastPacketAt(t *testing.T) {
+	db := setupTestDB(t)
+	defer db.Close()
+	seedTestData(t, db)
+
+	// Set last_packet_at for obs1
+	ts := "2026-04-24T12:00:00Z"
+	db.conn.Exec(`UPDATE observers SET last_packet_at = ? WHERE id = ?`, ts, "obs1")
+
+	// Verify via GetObservers
+	observers, err := db.GetObservers()
+	if err != nil {
+		t.Fatal(err)
+	}
+	var obs1 *Observer
+	for i := range observers {
+		if observers[i].ID == "obs1" {
+			obs1 = &observers[i]
+			break
+		}
+	}
+	if obs1 == nil {
+		t.Fatal("obs1 not found")
+	}
+	if obs1.LastPacketAt == nil || *obs1.LastPacketAt != ts {
+		t.Errorf("expected LastPacketAt=%s via GetObservers, got %v", ts, obs1.LastPacketAt)
+	}
+
+	// Verify via GetObserverByID
+	obs, err := db.GetObserverByID("obs1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if obs.LastPacketAt == nil || *obs.LastPacketAt != ts {
+		t.Errorf("expected LastPacketAt=%s via GetObserverByID, got %v", ts, obs.LastPacketAt)
+	}
 }
 
 func TestGetObserverByIDNotFound(t *testing.T) {
@@ -1109,7 +1156,8 @@ func setupTestDBV2(t *testing.T) *DB {
 			iata TEXT,
 			last_seen TEXT,
 			first_seen TEXT,
-			packet_count INTEGER DEFAULT 0
+			packet_count INTEGER DEFAULT 0,
+			last_packet_at TEXT DEFAULT NULL
 		);
 
 		CREATE TABLE transmissions (

public/observer-detail.js

@@ -150,6 +150,14 @@
           <div class="stat-label">First Seen</div>
           <div class="stat-value" style="font-size:0.85em">${obs.first_seen ? new Date(obs.first_seen).toLocaleDateString() : '—'}</div>
         </div>
+        <div class="stat-card">
+          <div class="stat-label">Last Status Update</div>
+          <div class="stat-value" style="font-size:0.85em">${obs.last_seen ? timeAgo(obs.last_seen) + '<br><span style="font-size:0.8em;color:var(--text-muted)">' + new Date(obs.last_seen).toLocaleString() + '</span>' : '—'}</div>
+        </div>
+        <div class="stat-card">
+          <div class="stat-label">Last Packet Observation</div>
+          <div class="stat-value" style="font-size:0.85em">${obs.last_packet_at ? timeAgo(obs.last_packet_at) + '<br><span style="font-size:0.8em;color:var(--text-muted)">' + new Date(obs.last_packet_at).toLocaleString() + '</span>' : '<span style="color:var(--text-muted)">never</span>'}</div>
+        </div>
       </div>
       <div class="mono" style="font-size:0.75em;color:var(--text-muted);margin-bottom:20px;word-break:break-all">
         ID: ${obs.id}

public/observers.js

@@ -75,6 +75,17 @@
     return { cls: 'health-red', label: 'Offline' };
   }
 
+  function packetBadge(o) {
+    if (!o.last_packet_at) return '<span title="No packets ever observed">📡⚠ never</span>';
+    const pktAgo = Date.now() - new Date(o.last_packet_at).getTime();
+    const statusAgo = o.last_seen ? Date.now() - new Date(o.last_seen).getTime() : Infinity;
+    const gap = pktAgo - statusAgo;
+    if (gap > 600000) {
+      return `<span title="Last packet ${timeAgo(o.last_packet_at)} — status is newer by ${Math.round(gap/60000)}min. Observer may be alive but not forwarding packets.">📡⚠ ${timeAgo(o.last_packet_at)}</span>`;
+    }
+    return timeAgo(o.last_packet_at);
+  }
+
   function uptimeStr(firstSeen) {
     if (!firstSeen) return '—';
     const ms = Date.now() - new Date(firstSeen).getTime();
@@ -123,7 +134,7 @@
       <div class="obs-table-scroll"><table class="data-table obs-table" id="obsTable">
         <caption class="sr-only">Observer status and statistics</caption>
         <thead><tr>
-          <th scope="col">Status</th><th scope="col">Name</th><th scope="col">Region</th><th scope="col">Last Seen</th>
+          <th scope="col">Status</th><th scope="col">Name</th><th scope="col">Region</th><th scope="col">Last Status</th><th scope="col">Last Packet</th>
           <th scope="col">Packets</th><th scope="col">Packets/Hour</th><th scope="col">Uptime</th>
         </tr></thead>
         <tbody>${filtered.map(o => {
@@ -134,6 +145,7 @@
             <td class="mono">${o.name || o.id}</td>
             <td>${o.iata ? `<span class="badge-region">${o.iata}</span>` : '—'}</td>
             <td>${timeAgo(o.last_seen)}</td>
+            <td>${packetBadge(o)}</td>
             <td>${(o.packet_count || 0).toLocaleString()}</td>
             <td>${sparkBar(o.packetsLastHour || 0, maxPktsHr)}</td>
             <td>${uptimeStr(o.first_seen)}</td>

public/style.css

@@ -1556,7 +1556,7 @@ tr[data-hops]:hover { background: rgba(59,130,246,0.1); }
 
 /* #20 — Observers table horizontal scroll on mobile */
 .obs-table-scroll { overflow-x: auto; -webkit-overflow-scrolling: touch; }
-.obs-table-scroll .obs-table { min-width: 640px; }
+.obs-table-scroll .obs-table { min-width: 720px; }
 
 /* #206 — Analytics/Compare tables scroll wrappers on mobile */
 .analytics-table-scroll { overflow-x: auto; -webkit-overflow-scrolling: touch; }