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/services/metadata.py

@@ -15,6 +15,7 @@ from sqlalchemy import select
 
 from app.database import AsyncSessionLocal
 from app.models import Photo
+from app.services.date_guess import has_date_warning
 
 logger = logging.getLogger(__name__)
 
@@ -238,6 +239,14 @@ async def _extract_metadata_async(photo_id: str):
                                 photo.taken_at = taken_at
                                 photo.taken_at_source = 'exif'
                                 break
+
+                    # Re-run the path-vs-date heuristic now that we know
+                    # whether EXIF provided a real capture date. A true EXIF
+                    # date that matches the folder clears the warning the
+                    # scanner set during the filesystem-mtime pass.
+                    photo.has_date_warning = has_date_warning(
+                        photo.filepath, photo.taken_at
+                    )
                     
                     # Extract dimensions if not already set
                     if not photo.width: