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/tasks/scan.py

@@ -21,6 +21,7 @@ from app.models import Photo, Folder, SourceRoot
 from app.config import settings
 from app.tasks.thumbs import generate_thumbnails
 from app.services.metadata import extract_metadata
+from app.services.date_guess import has_date_warning
 
 logger = logging.getLogger(__name__)
 
@@ -231,6 +232,7 @@ async def _scan_folder_async(folder_path: str, source_root_id: Optional[str], ta
                             effective_hidden = await is_folder_effectively_hidden(folder)
 
                             # Create photo entry
+                            mtime_dt = datetime.fromtimestamp(stat.st_mtime)
                             photo = Photo(
                                 filepath=filepath,
                                 filename=filename,
@@ -239,8 +241,13 @@ async def _scan_folder_async(folder_path: str, source_root_id: Optional[str], ta
                                 media_type=get_media_type(filepath),
                                 original_format=Path(filepath).suffix.upper()[1:],
                                 file_size=stat.st_size,
-                                taken_at=datetime.fromtimestamp(stat.st_mtime),
+                                taken_at=mtime_dt,
                                 taken_at_source='filesystem',
+                                # First-pass flag based on the filesystem mtime;
+                                # metadata.extract_metadata re-runs this once
+                                # EXIF has been parsed so a real DateTimeOriginal
+                                # can clear the warning.
+                                has_date_warning=has_date_warning(filepath, mtime_dt),
                                 is_duplicate=is_dup,
                                 is_hidden=effective_hidden,
                                 processing_status='pending'