fix(external): path-safe unlink + per-link staging + orphan repair (#859)

External downloads import IN PLACE, so the post-attach dedup-skip unlink could
delete a file that IS an ImageRecord's backing file — orphaning the record and
404-ing on playback. Two sources of that:

- Two links on the same post (same film from mega + gdrive) emitted the same
  filename into one external/<post_id>/ dir; the second overwrote the first.
  Stage per-LINK now (external/<post_id>/<link_id>/) so each file keeps its path.
- The duplicate_hash/duplicate_phash branch unlinked `f` unconditionally. Make it
  path-safe: only unlink when `f` is NOT the existing record's canonical file.

Plus an operator-triggered orphan-repair maintenance task
(prune_missing_file_records_task) to clean up records already orphaned by the
bug: scans ImageRecords, deletes those whose file is gone (cascade), with an
NFS-stall guard that aborts without deleting if a large sample is mostly missing.
Wired through POST /api/admin/maintenance/prune-missing-files and a
MissingFileRepairCard in the Maintenance panel.

Tests: refetch-same-link keeps the canonical file; orphan repair deletes only
real orphans and aborts on the mostly-missing guard.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/api/admin.py

@@ -348,3 +348,16 @@ async def trigger_reextract_archives():
 
     async_result = reextract_archive_attachments_task.delay()
     return jsonify({"task_id": async_result.id, "status": "queued"}), 202
+
+
+@admin_bp.route("/maintenance/prune-missing-files", methods=["POST"])
+async def trigger_prune_missing_files():
+    """Operator-triggered orphan repair (#859): delete ImageRecords whose backing
+    file is gone from disk (e.g. left by the external-attach unlink bug), so they
+    stop 404-ing on playback. The task aborts WITHOUT deleting if a large fraction
+    of files look missing (a filesystem/NFS stall). Maintenance queue;
+    operator-triggered only — never an unattended sweep."""
+    from ..tasks.admin import prune_missing_file_records_task
+
+    async_result = prune_missing_file_records_task.delay()
+    return jsonify({"task_id": async_result.id, "status": "queued"}), 202