fix: photo upload date parameter parsing

Problem: Photos were always getting NULL date instead of form date,
causing frontend to fallback to created timestamp (today).

Root cause: FastAPI requires Form() wrapper for form fields when
mixing with File() parameters. Without it, the date parameter was
treated as query parameter and always received empty string.

Solution:
- Import Form from fastapi
- Change date parameter from str="" to str=Form("")
- Return photo_date instead of date in response (consistency)

Now photos correctly use the date from the upload form and can be
backdated when uploading later.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/routers/photos.py

@@ -9,7 +9,7 @@ import logging
 from pathlib import Path
 from typing import Optional
 
-from fastapi import APIRouter, UploadFile, File, Header, HTTPException, Depends
+from fastapi import APIRouter, UploadFile, File, Form, Header, HTTPException, Depends
 from fastapi.responses import FileResponse
 import aiofiles
 
@@ -26,7 +26,7 @@ PHOTOS_DIR.mkdir(parents=True, exist_ok=True)
 
 
 @router.post("")
-async def upload_photo(file: UploadFile=File(...), date: str="",
+async def upload_photo(file: UploadFile=File(...), date: str=Form(""),
                        x_profile_id: Optional[str]=Header(default=None), session: dict=Depends(require_auth)):
     """Upload progress photo."""
     pid = get_pid(x_profile_id)
@@ -62,7 +62,7 @@ async def upload_photo(file: UploadFile=File(...), date: str="",
     # Phase 2: Increment usage counter
     increment_feature_usage(pid, 'photos')
 
-    return {"id":fid,"date":date}
+    return {"id":fid,"date":photo_date}
 
 
 @router.get("/{fid}")