fix: a post record is marked seen only after it is upserted, and an hourly sweep dates the posts killed walks left undated (#4436)

ingest_core marked a post's record key seen when it wrote _post.json, but
the record reaches the database (and dates the post and its images) only
in phase 3. A walk killed before phase 3 left posts the ledger called
recorded and the database never dated; ticks early-out long before
reaching them again. Keys now join the media in mark_seen_after_import.

date_posts_from_records finds each undated native post's record under
its artist's folder and upserts it with the post's own source. Hourly on
maintenance_long; an empty query once everything is dated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

tests/test_celery_routing.py

@@ -28,6 +28,7 @@ def test_quick_maintenance_stays_on_maintenance():
     ("backend.app.tasks.translation.translate_posts", "maintenance_long"),
     ("backend.app.tasks.gpu_queue.enqueue_gpu_backfill", "maintenance"),
     ("backend.app.tasks.maintenance.backfill_phash", "maintenance_long"),
+    ("backend.app.tasks.maintenance.date_posts_from_records", "maintenance_long"),
     ("backend.app.tasks.admin.normalize_tags_task", "maintenance_long"),
     ("backend.app.tasks.backup.backup_db_task", "maintenance_long"),
     ("backend.app.tasks.maintenance.vacuum_analyze", "maintenance"),