Add teams migration to runtime migrations

Add teams table, team_memberships table, and team_id column to projects
in the runtime migrations. This fixes startup failures on existing databases
that don't have the teams schema from the migration file.

backend/app/database.py

@@ -347,6 +347,7 @@ def _run_migrations():
                 conn.execute(text(migration))
                 conn.commit()
             except Exception as e:
+                conn.rollback()
                 logger.warning(f"Migration failed (may already be applied): {e}")