fix(metadata): drop EXIF:ModifyDate fallback, prefer SubSec, fall back to path

The taken_at extractor walked four EXIF fields in order: DateTimeOriginal,
CreateDate, MediaCreateDate, ModifyDate. The last one is set every time
a file is re-saved (Lightroom export, EXIF strip, batch resize), so any
photo whose original capture metadata was lost during editing ended up
labeled 'exif' with the *edit* date instead of the shoot date.

Changes:
  - SubSecDateTimeOriginal at the top of the list (sub-second precision,
    often carries OffsetTime).
  - QuickTime:CreateDate added next to MediaCreateDate.
  - ModifyDate dropped from the trusted list entirely.
  - When no trusted EXIF date is present, fall back to guess_date_from_path
    (already used for has_date_warning) and tag taken_at_source='path'.
    Better than filesystem mtime, which on Nextcloud-mounted libraries
    just reflects the upload time.
  - Skip the date-write block entirely if photo.taken_at_source == 'manual'
    so a rescan can't clobber a user correction.
  - parse_exif_datetime: handle the all-zero placeholder some cameras
    emit, accept tz-aware variants (%z), normalize to naive UTC.

Frontend: new 'PATH' badge in TakenAtEditor with a tooltip explaining
the date came from filename / folder rather than real EXIF.

Backfill: new backfill_taken_at celery task and
POST /api/v1/library/maintenance/backfill-taken-at endpoint that
re-enqueues extract_metadata for every non-manual photo. ~21k tasks
finish in ~15 min on the existing worker-light concurrency.

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

backend/app/routers/library.py

@@ -187,6 +187,19 @@ async def trigger_backfill_gps(current_user: User = Depends(get_current_user)):
     backfill_gps.delay()
     return {"status": "success", "message": "GPS backfill queued"}
 
+
+@router.post("/maintenance/backfill-taken-at")
+async def trigger_backfill_taken_at(current_user: User = Depends(get_current_user)):
+    """Re-run extract_metadata on every non-manual photo to recompute
+    taken_at with the current EXIF-priority list and path-based fallback.
+    Useful after the date-extraction logic changes (e.g. dropping the
+    ModifyDate fallback). Manual edits are preserved."""
+    from app.services.metadata import backfill_taken_at
+
+    backfill_taken_at.delay()
+    return {"status": "success", "message": "taken_at backfill queued"}
+
+
 @router.get("/scan/status")
 async def get_scan_status(db: AsyncSession = Depends(get_db), current_user: User = Depends(get_current_user)):
     """Get current scan status"""