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/service.py

@@ -93,33 +93,39 @@ class ScheduleItemService(BaseService):
     ) -> ScheduleItemResponse:
         user_id = self.require_user_id()
 
-        existing = await self._repository.get_by_item_id(item_id, user_id)
-        if existing is None:
-            raise HTTPException(status_code=404, detail="Schedule item not found")
-
-        update_data: dict = {}
-        if request.title is not None:
-            update_data["title"] = request.title
-        if request.description is not None:
-            update_data["description"] = request.description
-        if request.start_at is not None:
-            update_data["start_at"] = request.start_at
-        if request.end_at is not None:
-            update_data["end_at"] = request.end_at
-        if request.timezone is not None:
-            update_data["timezone"] = request.timezone
-        if request.status is not None:
-            update_data["status"] = request.status
-        if request.metadata is not None:
-            update_data["metadata"] = request.metadata.model_dump()
-
-        if request.end_at and request.start_at and request.end_at <= request.start_at:
-            raise HTTPException(status_code=400, detail="end_at must be after start_at")
-
-        if not update_data:
-            return self._to_response(existing)
-
         try:
+            existing = await self._repository.get_by_item_id(item_id, user_id)
+            if existing is None:
+                raise HTTPException(status_code=404, detail="Schedule item not found")
+
+            update_data: dict = {}
+            if request.title is not None:
+                update_data["title"] = request.title
+            if request.description is not None:
+                update_data["description"] = request.description
+            if request.start_at is not None:
+                update_data["start_at"] = request.start_at
+            if request.end_at is not None:
+                update_data["end_at"] = request.end_at
+            if request.timezone is not None:
+                update_data["timezone"] = request.timezone
+            if request.status is not None:
+                update_data["status"] = request.status
+            if request.metadata is not None:
+                update_data["metadata"] = request.metadata.model_dump()
+
+            next_start = (
+                request.start_at if request.start_at is not None else existing.start_at
+            )
+            next_end = request.end_at if request.end_at is not None else existing.end_at
+            if next_end is not None and next_end <= next_start:
+                raise HTTPException(
+                    status_code=400, detail="end_at must be after start_at"
+                )
+
+            if not update_data:
+                return self._to_response(existing)
+
             item = await self._repository.update_by_item_id(
                 item_id, user_id, update_data
             )
@@ -139,11 +145,11 @@ class ScheduleItemService(BaseService):
     async def delete(self, item_id: UUID) -> None:
         user_id = self.require_user_id()
 
-        existing = await self._repository.get_by_item_id(item_id, user_id)
-        if existing is None:
-            raise HTTPException(status_code=404, detail="Schedule item not found")
-
         try:
+            existing = await self._repository.get_by_item_id(item_id, user_id)
+            if existing is None:
+                raise HTTPException(status_code=404, detail="Schedule item not found")
+
             await self._repository.delete_by_item_id(item_id, user_id)
             await self._session.commit()
         except SQLAlchemyError: