feat: editable taken_at + folder-based date repair and filter

Lets operators fix corrupted capture dates at scale. Adds an editable
Date Taken field with a folder/filename-derived suggestion hint, a bulk
Date Taken section in the multi-select sidebar that either applies one
date to the whole selection or infers a per-photo date from each path,
a warning badge on thumbnails whose stored date disagrees with the
path, and a "Date issues" filter pill so suspicious photos can be
surfaced and fixed as a group. Edits are written back to EXIF on disk
so rescans don't clobber the fix.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/schemas/photos.py

@@ -39,6 +39,7 @@ class PhotoResponse(PhotoBase):
     latitude: Optional[float] = None
     longitude: Optional[float] = None
     is_duplicate: bool = False
+    has_date_warning: bool = False
     live_photo_video_id: Optional[str] = None
     # tags: List[Dict[str, Any]] = []  # TODO: Enable when using eager loading