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>

frontend/src/components/settings/MaintenancePanel.vue

@@ -16,6 +16,7 @@
     <AliasTable class="mt-4" />
     <DbMaintenanceCard class="mt-6" />
     <ArchiveReextractCard class="mt-6" />
+    <MissingFileRepairCard class="mt-6" />
     <BackupCard class="mt-6" />
     <!-- TagMaintenanceCard moved to Cleanup tab (v26.05.25.7) — it
          operates on the existing library which fits the Cleanup-tab
@@ -35,6 +36,7 @@ import AllowlistTable from './AllowlistTable.vue'
 import AliasTable from './AliasTable.vue'
 import DbMaintenanceCard from './DbMaintenanceCard.vue'
 import ArchiveReextractCard from './ArchiveReextractCard.vue'
+import MissingFileRepairCard from './MissingFileRepairCard.vue'
 import BackupCard from './BackupCard.vue'
 import { useSystemActivityStore } from '../../stores/systemActivity.js'