fix: update observer last_seen on packet ingestion (#479)

## Summary

Related to #463 (partial fix — addresses packet path, status message
path still needs investigation) — Observers incorrectly showing as
offline despite actively forwarding packets.

## Root Cause

Observer `last_seen` was only updated when status topic messages
(`meshcore/<region>/<observer_id>/status`) were received via
`UpsertObserver`. When packets were ingested from an observer, the
observer's `last_seen` was **not** updated — only the `observer_idx` was
resolved for the observation record.

This meant observers with low traffic that published status messages
less frequently than the 10-minute online threshold would appear offline
on the observers page, even though they were clearly alive and
forwarding packets.

## Changes

**`cmd/ingestor/db.go`:**
- Added `stmtUpdateObserverLastSeen` prepared statement: `UPDATE
observers SET last_seen = ? WHERE rowid = ?`
- In `InsertTransmission`, after resolving `observer_idx`, update the
observer's `last_seen` to the packet timestamp
- This ensures any observer actively forwarding traffic stays marked as
online

**`cmd/ingestor/db_test.go`:**
- Added `TestInsertTransmissionUpdatesObserverLastSeen` — verifies that
inserting a packet from an observer updates its `last_seen` from a
backdated value to the packet timestamp

## Performance

The added `UPDATE` is a single-row update by `rowid` (primary key) —
O(1) with no index overhead. It runs once per packet insertion when an
observer is resolved, which was already doing a `SELECT` by `rowid`
anyway. No measurable impact on ingestion throughput.

## Test Results

All existing tests pass:
- `cmd/ingestor`: 26.6s ✅
- `cmd/server`: 3.7s ✅

---------

Co-authored-by: you <you@example.com>

cmd/ingestor/db.go

@@ -36,8 +36,9 @@ type Store struct {
 	stmtUpsertNode           *sql.Stmt
 	stmtIncrementAdvertCount *sql.Stmt
 	stmtUpsertObserver       *sql.Stmt
-	stmtGetObserverRowid     *sql.Stmt
-	stmtUpdateNodeTelemetry  *sql.Stmt
+	stmtGetObserverRowid       *sql.Stmt
+	stmtUpdateObserverLastSeen *sql.Stmt
+	stmtUpdateNodeTelemetry    *sql.Stmt
 }
 
 // OpenStore opens or creates a SQLite DB at the given path, applying the
@@ -369,6 +370,11 @@ func (s *Store) prepareStatements() error {
 		return err
 	}
 
+	s.stmtUpdateObserverLastSeen, err = s.db.Prepare("UPDATE observers SET last_seen = ? WHERE rowid = ?")
+	if err != nil {
+		return err
+	}
+
 	s.stmtUpdateNodeTelemetry, err = s.db.Prepare(`
 		UPDATE nodes SET
 			battery_mv = COALESCE(?, battery_mv),
@@ -428,13 +434,16 @@ func (s *Store) InsertTransmission(data *PacketData) (bool, error) {
 		s.Stats.DuplicateTransmissions.Add(1)
 	}
 
-	// Resolve observer_idx
+	// Resolve observer_idx and update last_seen
 	var observerIdx *int64
 	if data.ObserverID != "" {
 		var rowid int64
 		err := s.stmtGetObserverRowid.QueryRow(data.ObserverID).Scan(&rowid)
 		if err == nil {
 			observerIdx = &rowid
+			// Update observer last_seen on every packet to prevent
+			// low-traffic observers from appearing offline (#463)
+			_, _ = s.stmtUpdateObserverLastSeen.Exec(now, rowid)
 		}
 	}
 

cmd/ingestor/db_test.go

@@ -516,6 +516,56 @@ func TestInsertTransmissionWithObserver(t *testing.T) {
 	}
 }
 
+// #463: Verify that inserting a packet updates the observer's last_seen,
+// so low-traffic observers don't incorrectly appear offline.
+func TestInsertTransmissionUpdatesObserverLastSeen(t *testing.T) {
+	s, err := OpenStore(tempDBPath(t))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer s.Close()
+
+	// Insert observer with an old last_seen
+	if err := s.UpsertObserver("obs1", "Observer1", "SJC", nil); err != nil {
+		t.Fatal(err)
+	}
+	// Backdate last_seen to 2 hours ago
+	oldTime := "2026-03-24T22:00:00Z"
+	s.db.Exec("UPDATE observers SET last_seen = ? WHERE id = ?", oldTime, "obs1")
+
+	// Verify it was backdated
+	var lastSeenBefore string
+	s.db.QueryRow("SELECT last_seen FROM observers WHERE id = ?", "obs1").Scan(&lastSeenBefore)
+	if lastSeenBefore != oldTime {
+		t.Fatalf("expected last_seen=%s, got %s", oldTime, lastSeenBefore)
+	}
+
+	// Insert a packet from this observer
+	data := &PacketData{
+		RawHex:      "0A00D69F",
+		Timestamp:   "2026-03-25T01:00:00Z",
+		ObserverID:  "obs1",
+		Hash:        "lastseentest123456",
+		RouteType:   2,
+		PayloadType: 2,
+		PathJSON:    "[]",
+		DecodedJSON: `{"type":"TXT_MSG"}`,
+	}
+	if _, err := s.InsertTransmission(data); err != nil {
+		t.Fatal(err)
+	}
+
+	// Verify last_seen was updated
+	var lastSeenAfter string
+	s.db.QueryRow("SELECT last_seen FROM observers WHERE id = ?", "obs1").Scan(&lastSeenAfter)
+	if lastSeenAfter == oldTime {
+		t.Error("observer last_seen was NOT updated after packet insertion — low-traffic observers will appear offline")
+	}
+	if lastSeenAfter != "2026-03-25T01:00:00Z" {
+		t.Errorf("expected last_seen=2026-03-25T01:00:00Z, got %s", lastSeenAfter)
+	}
+}
+
 func TestEndToEndIngest(t *testing.T) {
 	s, err := OpenStore(tempDBPath(t))
 	if err != nil {