refactor: update migration paths and improve database operation handling

backend/app/db/sqlmodel_exec.py

@@ -0,0 +1,9 @@
+from __future__ import annotations
+
+from sqlalchemy.sql.base import Executable
+from sqlmodel.ext.asyncio.session import AsyncSession
+
+
+async def exec_dml(session: AsyncSession, statement: Executable) -> None:
+    # SQLModel's AsyncSession typing only overloads exec() for SELECT statements.
+    await session.exec(statement)  # type: ignore[call-overload]