fix(photos): coerce tz-aware taken_at to naive UTC before DB write

PATCH /api/v1/photos/{id} returned 500 with
'can't subtract offset-naive and offset-aware datetimes' when the
frontend sent a tz-aware taken_at value (e.g. 2026-05-09T00:12+02:00).
The photos.taken_at column is timestamp without time zone, so asyncpg
refuses to bind a tz-aware datetime.

The frontend's datetime-local input is supposed to be naive but real-
world locales / browsers / paste flows occasionally include offsets.
Normalize on the server: if tzinfo is present, convert to UTC and drop
the tzinfo so both shapes round-trip cleanly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/routers/photos.py

@@ -2,7 +2,7 @@
 Photos API router
 """
 from typing import List, Optional, Dict, Any
-from datetime import datetime
+from datetime import datetime, timezone
 from pathlib import Path
 from fastapi import APIRouter, Depends, HTTPException, Query, Response
 from fastapi.responses import FileResponse
@@ -888,6 +888,15 @@ async def update_photo(
     if 'taken_at' in update_data:
         new_dt = update_data.pop('taken_at')
         if new_dt is not None:
+            # The frontend's <input type="datetime-local"> usually serializes
+            # without a tz, but a manual edit / paste / certain locales can
+            # send a tz-aware ISO (e.g. "2026-05-09T00:12+02:00"). The
+            # photos.taken_at column is `timestamp without time zone`, so
+            # asyncpg can't bind a tz-aware value — it raises
+            # "can't subtract offset-naive and offset-aware datetimes".
+            # Normalize to naive UTC so both shapes round-trip cleanly.
+            if new_dt.tzinfo is not None:
+                new_dt = new_dt.astimezone(timezone.utc).replace(tzinfo=None)
             try:
                 await write_taken_at(photo.filepath, new_dt)
             except ExifWriteError as exc: