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>

backend/app/services/patreon_downloader.py

@@ -50,10 +50,11 @@ _TITLE_MAX = 40
 _TIMEOUT_SECONDS = 120.0
 _CHUNK = 1 << 16
 
-# Same Referer/Origin gallery-dl forwards to yt-dlp for Mux-hosted Patreon
-# video (gallery_dl.py _get_default_config -> downloader.ytdl.raw-options).
-# Mux's JWT playback policy checks Referer/Origin on every request, so yt-dlp
-# must send Patreon's, not its own default.
+# Referer/Origin yt-dlp must send for Mux-hosted Patreon video. Mux's JWT
+# playback policy checks Referer/Origin on every request, so yt-dlp must send
+# Patreon's, not its own default. (gallery-dl forwarded the same headers before
+# the #697 cutover removed its Patreon path; this is now the only place they
+# live.)
 _VIDEO_HEADERS = {
     "Referer": "https://www.patreon.com/",
     "Origin": "https://www.patreon.com",