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/download_service.py

@@ -44,11 +44,14 @@ from .scheduler_service import set_platform_cooldown
 log = logging.getLogger(__name__)
 
 
+# Vanity → campaign-id resolution is still needed by the native Patreon
+# ingester (phase 2 resolves the campaign id before the walk). gallery-dl's
+# reactive campaign-id retry + the `id:` effective-URL rewrite were removed at
+# the #697 cutover (Patreon no longer flows through gallery-dl).
 _PATREON_VANITY_RE = re.compile(
     r"^https?://(?:www\.)?patreon\.com/(?:c/)?(?!id:)([^/?#]+)",
     re.IGNORECASE,
 )
-_CAMPAIGN_ID_FAILURE_PATTERN = "failed to extract campaign id"
 
 
 def _extract_patreon_vanity(url: str) -> str | None:
@@ -56,16 +59,6 @@ def _extract_patreon_vanity(url: str) -> str | None:
     return m.group(1) if m else None
 
 
-def _looks_like_campaign_id_failure(stdout: str, stderr: str) -> bool:
-    return _CAMPAIGN_ID_FAILURE_PATTERN in f"{stdout}\n{stderr}".lower()
-
-
-def _effective_url(platform: str, source_url: str, overrides: dict) -> str:
-    if platform == "patreon" and overrides.get("patreon_campaign_id"):
-        return f"https://www.patreon.com/id:{overrides['patreon_campaign_id']}"
-    return source_url
-
-
 class DownloadService:
     """Async orchestrator. The Celery task runs `asyncio.run(svc.download_source(N))`.
 
@@ -160,11 +153,8 @@ class DownloadService:
                 ctx, source_config, mode,
             )
         else:
-            effective_url = _effective_url(
-                ctx["platform"], ctx["url"], ctx["config_overrides"] or {}
-            )
             dl_result = await self.gdl.download(
-                url=effective_url,
+                url=ctx["url"],
                 artist_slug=ctx["artist_slug"],
                 platform=ctx["platform"],
                 source_config=source_config,