fix: photo upload with empty date string

Problem:
- Photo upload with empty date parameter (date='')
- PostgreSQL rejects empty string for DATE field
- Error: "invalid input syntax for type date: ''"
- Occurred when saving circumference entry with only photo

Fix:
- Convert empty string to NULL before INSERT
- Check: date if date and date.strip() else None
- NULL is valid for optional date field

Test case:
- Circumference entry with only photo → should work now
- Photo without date → stored with date=NULL ✓

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

backend/routers/photos.py

@@ -50,10 +50,14 @@ async def upload_photo(file: UploadFile=File(...), date: str="",
     ext = Path(file.filename).suffix or '.jpg'
     path = PHOTOS_DIR / f"{fid}{ext}"
     async with aiofiles.open(path,'wb') as f: await f.write(await file.read())
+
+    # Convert empty string to NULL for date field
+    photo_date = date if date and date.strip() else None
+
     with get_db() as conn:
         cur = get_cursor(conn)
         cur.execute("INSERT INTO photos (id,profile_id,date,path,created) VALUES (%s,%s,%s,%s,CURRENT_TIMESTAMP)",
-                     (fid,pid,date,str(path)))
+                     (fid,pid,photo_date,str(path)))
 
     # Phase 2: Increment usage counter
     increment_feature_usage(pid, 'photos')