feat(patreon): recovery UI + gallery-dl cutover — build step 5 (plan #697)

Final step of the native Patreon ingester: a first-class Recovery action,
and removal of the now-dead gallery-dl Patreon path.

Recovery (rules #23/#24/#27 — full product, with UI):
- source_service.start_recovery arms the #693 backfill state machine PLUS
  `_backfill_bypass_seen`, flipping download mode to recovery (bypass the
  seen-ledger to re-fetch dropped-and-deleted near-dups and re-evaluate
  under the current pHash threshold). Stop via the shared stop_backfill.
- SourceRecord exposes backfill_bypass_seen; POST /sources/{id}/backfill
  gains action="recover".
- Frontend: Recovery button (Patreon-only, mdi-backup-restore) on SourceRow
  + SourceCard; the running badge labels "Recovering (N)" vs "Backfilling
  (N)"; the Stop tooltip says "Stop recovery". sources.js recoverSource +
  SubscriptionsTab onRecover.

Cutover (rule #22 — no legacy):
- gallery_dl: removed PLATFORM_DEFAULTS["patreon"], the patreon
  files/cursor branch in _build_config_for_source, and the patreon/Mux
  yt-dlp Referer/Origin block (was patreon-specific and wrongly tagged the
  other platforms' yt-dlp fetches; native ingester owns it now).
- download_service: removed the dead campaign-id-retry helpers
  (_looks_like_campaign_id_failure / _CAMPAIGN_ID_FAILURE_PATTERN) and
  _effective_url. Vanity→campaign resolution + resume_cursor + the
  cursor/PARTIAL lifecycle stay — they serve the native ingester.

Tests: removed the two obsolete patreon-gallery-dl config tests (yt-dlp
Referer, resume-cursor); repointed the generic skip-value config tests to
subscribestar; added start_recovery + recover-endpoint coverage
(backfill_bypass_seen). gallery-dl stays for the other 5 platforms.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/stores/sources.js

@@ -98,6 +98,14 @@ export const useSourcesStore = defineStore('sources', () => {
     _invalidate(artistIdHint ?? body.artist_id)
     return body
   }
+  // Plan #697: arm a recovery walk — a backfill that bypasses the Patreon
+  // seen-ledger to re-fetch dropped-and-deleted near-dups and re-evaluate them
+  // under the current pHash threshold. Stop with stopBackfill (shared lifecycle).
+  async function recoverSource(id, artistIdHint = null) {
+    const body = await api.post(`/api/sources/${id}/backfill`, { body: { action: 'recover' } })
+    _invalidate(artistIdHint ?? body.artist_id)
+    return body
+  }
 
   function sourcesByArtistGrouped() {
     // returns [{artist: {id,name,slug}, sources: [...]}, ...]
@@ -127,6 +135,7 @@ export const useSourcesStore = defineStore('sources', () => {
     checkNow,
     startBackfill,
     stopBackfill,
+    recoverSource,
     findOrCreateArtist, autocompleteArtist,
     loadScheduleStatus,
     sourcesByArtistGrouped,