fix(backend): alembic migration SQLite compat + sync push rollback on error

backend/app/sync/service.py

@@ -130,6 +130,7 @@ async def apply_push(
                 applied += 1
         except Exception as exc:  # noqa: BLE001
             errors.append({"table": table, "id": op.get("id"), "error": str(exc)})
+            await db.rollback()
 
     await db.commit()
     return {"applied": applied, "conflicts": errors}