feat: Cleanup can import files that are on disk but not in the library

A download writes its files into the artist's folder and imports them in a
later step. A run that stopped in between left complete files no record
points at, and a walk that had already marked them seen never returned for
them; on the live instance that is about 95 GB of art.

"Import files the library is missing" (Cleanup, Duplicates & leftovers)
walks every folder named for an artist, matched by slug so a differently
cased folder still matches, and lists the images, videos and archives no
image, attachment or queued import points at. Stores, sidecars, staging
files and files changed in the last day are left alone. Preview shows the
counts per artist; Import queues the same list as a library import batch,
whose tasks file the folder's post record, attach each file where it lies
with its folder's artist and source, and remove the ones the library
already holds or the import filters reject. A file whose import crashed is
kept.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/api/admin.py

@@ -475,6 +475,21 @@ async def trigger_reclaim_attachments():
     return _queued(async_result)
 
 
+@admin_bp.route("/maintenance/import-unimported", methods=["POST"])
+async def trigger_import_unimported():
+    """Import files in the artists' folders that the library has no record of.
+    Body {"dry_run": bool}; dry_run (the DEFAULT) lists what was found, per
+    artist, and changes nothing. dry_run=false queues them as an import batch
+    that attaches each file where it lies. Returns the Celery task id — poll
+    /maintenance/task-result/<id> for the summary."""
+    from ..tasks.admin import import_unimported_files_task
+
+    body = await request.get_json(silent=True) or {}
+    dry_run = bool(body.get("dry_run", True))
+    async_result = import_unimported_files_task.delay(dry_run=dry_run)
+    return _queued(async_result)
+
+
 @admin_bp.route("/maintenance/repair-discord-downloads", methods=["POST"])
 async def trigger_repair_discord_downloads():
     """Clean re-download of the Discord files broken by the `None` naming