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/models/photos.py

@@ -46,6 +46,13 @@ class Photo(Base):
     # column is maintained by two places: the scanner sets it on new
     # rows, and POST /folders/{id}/hide recomputes it on toggle.
     is_hidden = Column(Boolean, nullable=False, default=False, server_default='false', index=True)
+
+    # "Capture date is probably wrong" — denormalized from the folder/filename
+    # date-guesser. Set at scan time and recomputed on every taken_at edit so
+    # the filter bar can query it directly. See services/date_guess.py for
+    # the heuristic; kept as a stored column because recomputing on every
+    # list query would mean running the regex stack across thousands of rows.
+    has_date_warning = Column(Boolean, nullable=False, default=False, server_default='false', index=True)
     
     # Thumbnail paths
     thumb_small = Column(String)  # path to 240px thumb