feat: integrate auto-photo-save into item creation endpoint

- Extend ItemCreate schema with optional extracted_image_bytes (base64) and image_processing (dict)
- Update create_item endpoint to call _auto_save_photo_from_extraction after item creation
- Decode base64 image bytes and pass crop_bounds, rotation_degrees to helper
- Don't block item creation if photo save fails (log warning instead)
- Item returned with photo_path, photo_thumbnail_path populated if save succeeded
- Full backward compatibility: old clients without image fields work unchanged
- Add 5 integration tests covering all scenarios:
  - Create item WITH image_processing → photo auto-saved
  - Create item WITHOUT image_processing → no photo (backward compatible)
  - Create item WITH invalid image_processing → item created, photo skipped
  - Create item WITH crop_bounds=None → item created, photo skipped
  - Create item WITH bytes but NO processing metadata → item created, photo skipped
- All 158 backend tests passing, zero regressions

backend/schemas/items.py

@@ -1,5 +1,5 @@
 from pydantic import BaseModel, field_serializer
-from typing import Optional
+from typing import Optional, Dict, Any
 from datetime import datetime
 
 
@@ -69,7 +69,8 @@ class ItemBase(BaseModel):
 
 
 class ItemCreate(ItemBase):
-    pass
+    extracted_image_bytes: Optional[str] = None  # Base64-encoded image data from AI extraction
+    image_processing: Optional[Dict[str, Any]] = None  # {crop_bounds, rotation_degrees, confidence} from AI
 
 
 class Item(ItemBase):