perf: query only matching node locations in handleObservers (#579)

## Summary

`handleObservers()` in `routes.go` was calling `GetNodeLocations()`
which fetches ALL nodes from the DB just to match ~10 observer IDs
against node public keys. With 500+ nodes this is wasteful.

## Changes

- **`db.go`**: Added `GetNodeLocationsByKeys(keys []string)` — queries
only the rows matching the given public keys using a parameterized
`WHERE LOWER(public_key) IN (?, ?, ...)` clause.
- **`routes.go`**: `handleObservers` now collects observer IDs and calls
the targeted method instead of the full-table scan.
- **`coverage_test.go`**: Added `TestGetNodeLocationsByKeys` covering
known key, empty keys, and unknown key cases.

## Performance

With ~10 observers and 500+ nodes, the query goes from scanning all 500
rows to fetching only ~10. The original `GetNodeLocations()` is
preserved for any other callers.

Fixes #378

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

cmd/server/coverage_test.go

@@ -1426,6 +1426,40 @@ func TestGetNodeLocations(t *testing.T) {
 	}
 }
 
+// --- GetNodeLocationsByKeys ---
+
+func TestGetNodeLocationsByKeys(t *testing.T) {
+	db := setupTestDB(t)
+	defer db.Close()
+	seedTestData(t, db)
+
+	// Query with a known key
+	pk := "aabbccdd11223344"
+	locs := db.GetNodeLocationsByKeys([]string{pk})
+	if len(locs) != 1 {
+		t.Errorf("expected 1 location, got %d", len(locs))
+	}
+	if entry, ok := locs[strings.ToLower(pk)]; ok {
+		if entry["lat"] == nil {
+			t.Error("expected non-nil lat")
+		}
+	} else {
+		t.Error("expected node location for test repeater")
+	}
+
+	// Query with no keys returns empty map
+	empty := db.GetNodeLocationsByKeys([]string{})
+	if len(empty) != 0 {
+		t.Errorf("expected 0 locations for empty keys, got %d", len(empty))
+	}
+
+	// Query with unknown key returns empty map
+	unknown := db.GetNodeLocationsByKeys([]string{"nonexistent"})
+	if len(unknown) != 0 {
+		t.Errorf("expected 0 locations for unknown key, got %d", len(unknown))
+	}
+}
+
 // --- Store edge cases ---
 
 func TestStoreQueryPacketsEdgeCases(t *testing.T) {

cmd/server/db.go

@@ -1497,6 +1497,39 @@ func (db *DB) GetNodeLocations() map[string]map[string]interface{} {
 	return result
 }
 
+// GetNodeLocationsByKeys returns location data only for the given public keys.
+// This avoids fetching ALL nodes when only a few keys need to be matched.
+func (db *DB) GetNodeLocationsByKeys(keys []string) map[string]map[string]interface{} {
+	result := make(map[string]map[string]interface{})
+	if len(keys) == 0 {
+		return result
+	}
+	placeholders := make([]string, len(keys))
+	args := make([]interface{}, len(keys))
+	for i, k := range keys {
+		placeholders[i] = "?"
+		args[i] = strings.ToLower(k)
+	}
+	query := "SELECT public_key, lat, lon, role FROM nodes WHERE LOWER(public_key) IN (" + strings.Join(placeholders, ",") + ")"
+	rows, err := db.conn.Query(query, args...)
+	if err != nil {
+		return result
+	}
+	defer rows.Close()
+	for rows.Next() {
+		var pk string
+		var role sql.NullString
+		var lat, lon sql.NullFloat64
+		rows.Scan(&pk, &lat, &lon, &role)
+		result[strings.ToLower(pk)] = map[string]interface{}{
+			"lat":  nullFloat(lat),
+			"lon":  nullFloat(lon),
+			"role": nullStr(role),
+		}
+	}
+	return result
+}
+
 // QueryMultiNodePackets returns transmissions referencing any of the given pubkeys.
 func (db *DB) QueryMultiNodePackets(pubkeys []string, limit, offset int, order, since, until string) (*PacketResult, error) {
 	if len(pubkeys) == 0 {

cmd/server/routes.go

@@ -1560,8 +1560,12 @@ func (s *Server) handleObservers(w http.ResponseWriter, r *http.Request) {
 	oneHourAgo := time.Now().Add(-1 * time.Hour).Unix()
 	pktCounts := s.db.GetObserverPacketCounts(oneHourAgo)
 
-	// Batch lookup: node locations (observer ID may match a node public_key)
-	nodeLocations := s.db.GetNodeLocations()
+	// Batch lookup: node locations only for observer IDs (not all nodes)
+	observerIDs := make([]string, len(observers))
+	for i, o := range observers {
+		observerIDs[i] = o.ID
+	}
+	nodeLocations := s.db.GetNodeLocationsByKeys(observerIDs)
 
 	result := make([]ObserverResp, 0, len(observers))
 	for _, o := range observers {