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

backend/app/tasks/maintenance.py

@@ -731,6 +731,32 @@ def recover_stalled_download_events() -> int:
         return events_recovered
 
 
+@celery.task(
+    name="backend.app.tasks.maintenance.date_posts_from_records",
+    soft_time_limit=1500,
+    time_limit=1800,
+)
+def date_posts_from_records() -> dict:
+    """Date undated native posts from the records their walk left on disk
+    (#4436). Hourly and self-limiting: once every post is dated it is one
+    empty query."""
+    from ..services.importer import Importer
+    from ..services.post_record_repair import date_posts_from_records as _repair
+    from ..services.thumbnailer import Thumbnailer
+
+    images_root = IMAGES_ROOT
+    SessionLocal = _sync_session_factory()
+    with SessionLocal() as session:
+        importer = Importer(
+            session=session,
+            images_root=images_root,
+            import_root=images_root,
+            thumbnailer=Thumbnailer(images_root=images_root),
+            settings=ImportSettings.load_sync(session),
+        )
+        return _repair(session, importer, images_root)
+
+
 @celery.task(name="backend.app.tasks.maintenance.recover_stalled_backup_runs")
 def recover_stalled_backup_runs() -> int:
     """Flip BackupRun rows stuck in running/restoring past the hard limit