test: add last_packet_at tests for ingestor and server

- Ingestor: verify last_packet_at is NULL after UpsertObserver (status path),
  set after InsertTransmission, and unchanged by subsequent UpsertObserver calls
- Server: verify last_packet_at reads back through GetObservers and GetObserverByID

cmd/ingestor/db_test.go

@@ -569,6 +569,59 @@ 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")
+	}
+	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_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 (