fix(api): expire MongoDB heartbeats after 60 days

Add a single-field TTL index on heartbeat timestamps while preserving the existing query index.

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

Copilot-Session: f27d8f52-9620-4ee2-a564-ec4870a5f840

Author: AchoArnold

api/pkg/repositories/mongodb.go

@@ -109,6 +109,12 @@ func createMongoIndexes(ctx context.Context, db *mongo.Database) error {
 
 	_, err := heartbeatsCol.Indexes().CreateMany(ctx, []mongo.IndexModel{
 		{Keys: bson.D{{Key: "user_id", Value: 1}, {Key: "owner", Value: 1}, {Key: "timestamp", Value: -1}}},
+		{
+			Keys: bson.D{{Key: "timestamp", Value: 1}},
+			Options: options.Index().
+				SetName("heartbeats_retention").
+				SetExpireAfterSeconds(int32((60 * 24 * time.Hour) / time.Second)),
+		},
 	})
 	if err != nil {
 		return stacktrace.Propagatef(err, "cannot create indexes on heartbeats collection")