fix: address code review issues and improve code quality

- Add owner_id check in repository delete operation
- Fix time range validation for partial updates
- Wrap pre-query in try/except for consistent error handling
- Use default_factory instead of mutable defaults
- Add max_length constraint for timezone field
- Remove unused dependencies and empty validators
- Extract magic numbers to constants
- Simplify update logic with model_dump

backend/src/v1/schedule_items/repository.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import TYPE_CHECKING, Protocol
 from uuid import UUID
 
@@ -95,7 +95,17 @@ class SQLAlchemyScheduleItemRepository(BaseRepository[ScheduleItem]):
         self, item_id: UUID, owner_id: UUID
     ) -> ScheduleItem | None:
         try:
-            return await self.soft_delete_by_id(item_id)
+            stmt = (
+                update(ScheduleItem)
+                .where(ScheduleItem.id == item_id)
+                .where(ScheduleItem.owner_id == owner_id)
+                .where(ScheduleItem.deleted_at.is_(None))
+                .values(deleted_at=datetime.now(timezone.utc))
+                .returning(ScheduleItem)
+            )
+            result = await self._session.execute(stmt)
+            await self._session.flush()
+            return result.scalar_one_or_none()
         except SQLAlchemyError:
             logger.exception("Schedule item delete failed", item_id=str(item_id))
             raise