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>

tests/test_maintenance.py

@@ -889,3 +889,14 @@ def test_reclaim_attachments_stuck_threshold_exceeds_hard_time_limit():
         "backend.app.tasks.admin.reclaim_orphaned_attachments_task"
     ]
     assert override >= hard_minutes
+
+
+def test_import_unimported_stuck_threshold_exceeds_hard_time_limit():
+    """The same invariant for the walk that imports files on disk."""
+    from backend.app.tasks.admin import import_unimported_files_task
+    from backend.app.tasks.maintenance import TASK_STUCK_THRESHOLD_MINUTES
+
+    override = TASK_STUCK_THRESHOLD_MINUTES[
+        "backend.app.tasks.admin.import_unimported_files_task"
+    ]
+    assert override >= import_unimported_files_task.time_limit / 60