Replace SQLModel with SQLAlchemy in items routes

- Replace Item.model_validate() with Item(**item_data, owner_id=...)
- Replace item.sqlmodel_update() with manual field updates using setattr()
- Maintain same functionality while removing SQLModel dependencies

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: sermetege

backend/app/api/routes/items.py

@@ -61,7 +61,8 @@ def create_item(
     """
     Create new item.
     """
-    item = Item.model_validate(item_in, update={"owner_id": current_user.id})
+    item_data = item_in.model_dump()
+    item = Item(**item_data, owner_id=current_user.id)
     session.add(item)
     session.commit()
     session.refresh(item)
@@ -85,7 +86,8 @@ def update_item(
     if not current_user.is_superuser and (item.owner_id != current_user.id):
         raise HTTPException(status_code=400, detail="Not enough permissions")
     update_dict = item_in.model_dump(exclude_unset=True)
-    item.sqlmodel_update(update_dict)
+    for field, value in update_dict.items():
+        setattr(item, field, value)
     session.add(item)
     session.commit()
     session.refresh(item)