fix(phase1): add photo fields to schemas and set datetime default

backend/schemas/items.py

@@ -1,5 +1,6 @@
-from pydantic import BaseModel
+from pydantic import BaseModel, field_serializer
 from typing import Optional
+from datetime import datetime
 
 
 # --- Categories ---
@@ -54,6 +55,9 @@ class ItemBase(BaseModel):
     image_url: Optional[str] = None
     box_label: Optional[str] = None
     labels_data: Optional[str] = None
+    photo_path: Optional[str] = None
+    photo_thumbnail_path: Optional[str] = None
+    photo_upload_date: Optional[datetime] = None
 
 
 class ItemCreate(ItemBase):
@@ -65,3 +69,8 @@ class Item(ItemBase):
 
     class Config:
         from_attributes = True
+
+    @field_serializer('photo_upload_date', when_used='json')
+    def serialize_photo_upload_date(self, value: Optional[datetime]) -> Optional[str]:
+        """Serialize datetime to ISO format string for JSON."""
+        return value.isoformat() if value else None