refactor: optimize heartbeats MongoDB indexes

Replace two separate indexes (owner_1_timestamp_-1, user_id_1) with a single
compound index {user_id: 1, owner: 1, timestamp: -1} that covers all query
patterns. Old indexes are dropped automatically on startup.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: AchoArnold

api/pkg/repositories/mongodb.go

@@ -106,9 +106,14 @@ func parseMongoDBName(uri string) (string, error) {
 func createMongoIndexes(ctx context.Context, db *mongo.Database) error {
 	// Heartbeats indexes
 	heartbeatsCol := db.Collection(collectionHeartbeats)
+
+	// TODO: Remove this block after deploying once — old indexes will have been dropped in production.
+	for _, name := range []string{"owner_1_timestamp_-1", "user_id_1"} {
+		_ = heartbeatsCol.Indexes().DropOne(ctx, name)
+	}
+
 	_, err := heartbeatsCol.Indexes().CreateMany(ctx, []mongo.IndexModel{
-		{Keys: bson.D{{"owner", 1}, {"timestamp", -1}}},
-		{Keys: bson.D{{"user_id", 1}}},
+		{Keys: bson.D{{"user_id", 1}, {"owner", 1}, {"timestamp", -1}}},
 	})
 	if err != nil {
 		return stacktrace.Propagate(err, "cannot create indexes on heartbeats collection")