fix: add ensureLastPacketAtColumn server-side migration

Mirror the ensureObserverInactiveColumn pattern (PR #961) for the
last_packet_at column added by the ingestor migration. Without this,
the server SELECTs last_packet_at but never adds it — causing 500
errors on /api/observers when running against DBs the ingestor has
not yet touched (e.g. the e2e fixture).

Adds TestEnsureLastPacketAtColumn for correctness + idempotency.

cmd/server/main.go

@@ -180,6 +180,12 @@ func main() {
 		log.Printf("[store] warning: could not add observers.inactive column: %v", err)
 	}
 
+	// Ensure observers.last_packet_at column exists (PR #905 reads it; ingestor migration
+	// adds it but server may run against DBs ingestor never touched, e.g. e2e fixture).
+	if err := ensureLastPacketAtColumn(dbPath); err != nil {
+		log.Printf("[store] warning: could not add observers.last_packet_at column: %v", err)
+	}
+
 	// Soft-delete observers that are in the blacklist (mark inactive=1) so
 	// historical data from a prior unblocked window is hidden too.
 	if len(cfg.ObserverBlacklist) > 0 {

cmd/server/neighbor_persist.go

@@ -320,6 +320,44 @@ func ensureObserverInactiveColumn(dbPath string) error {
 	return nil
 }
 
+// ensureLastPacketAtColumn adds the last_packet_at column to observers if missing.
+// The column was originally added by ingestor migration (observers_last_packet_at_v1)
+// to track the most recent packet observation time separately from status updates.
+// When the server starts against a DB that was never touched by the ingestor (e.g.
+// the e2e fixture), the column is missing and read queries that reference it
+// (GetObservers, GetObserverByID) fail with "no such column: last_packet_at".
+func ensureLastPacketAtColumn(dbPath string) error {
+	rw, err := openRW(dbPath)
+	if err != nil {
+		return err
+	}
+	defer rw.Close()
+
+	rows, err := rw.Query("PRAGMA table_info(observers)")
+	if err != nil {
+		return err
+	}
+	defer rows.Close()
+
+	for rows.Next() {
+		var cid int
+		var colName string
+		var colType sql.NullString
+		var notNull, pk int
+		var dflt sql.NullString
+		if rows.Scan(&cid, &colName, &colType, &notNull, &dflt, &pk) == nil && colName == "last_packet_at" {
+			return nil // already exists
+		}
+	}
+
+	_, err = rw.Exec("ALTER TABLE observers ADD COLUMN last_packet_at TEXT")
+	if err != nil {
+		return fmt.Errorf("add last_packet_at column: %w", err)
+	}
+	log.Println("[store] Added last_packet_at column to observers")
+	return nil
+}
+
 // softDeleteBlacklistedObservers marks observers matching the blacklist as
 // inactive=1 so they are hidden from API responses.  Runs once at startup.
 func softDeleteBlacklistedObservers(dbPath string, blacklist []string) {

cmd/server/neighbor_persist_test.go

@@ -538,3 +538,62 @@ func TestOpenRW_BusyTimeout(t *testing.T) {
 		t.Errorf("expected busy_timeout=5000, got %d", timeout)
 	}
 }
+
+func TestEnsureLastPacketAtColumn(t *testing.T) {
+	// Create a temp DB with observers table missing last_packet_at
+	dir := t.TempDir()
+	dbPath := dir + "/test.db"
+	db, err := sql.Open("sqlite", dbPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = db.Exec(`CREATE TABLE observers (
+		id TEXT PRIMARY KEY,
+		name TEXT,
+		last_seen TEXT,
+		lat REAL,
+		lon REAL,
+		inactive INTEGER DEFAULT 0
+	)`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	db.Close()
+
+	// First call: should add the column
+	if err := ensureLastPacketAtColumn(dbPath); err != nil {
+		t.Fatalf("first call failed: %v", err)
+	}
+
+	// Verify column exists
+	db2, err := sql.Open("sqlite", dbPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db2.Close()
+
+	var found bool
+	rows, err := db2.Query("PRAGMA table_info(observers)")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer rows.Close()
+	for rows.Next() {
+		var cid int
+		var colName string
+		var colType sql.NullString
+		var notNull, pk int
+		var dflt sql.NullString
+		if rows.Scan(&cid, &colName, &colType, &notNull, &dflt, &pk) == nil && colName == "last_packet_at" {
+			found = true
+		}
+	}
+	if !found {
+		t.Fatal("last_packet_at column not found after migration")
+	}
+
+	// Idempotency: second call should succeed without error
+	if err := ensureLastPacketAtColumn(dbPath); err != nil {
+		t.Fatalf("idempotent call failed: %v", err)
+	}
+}