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/api/sources.py

@@ -122,23 +122,30 @@ async def delete_source(source_id: int):
 
 @sources_bp.route("/<int:source_id>/backfill", methods=["POST"])
 async def set_backfill(source_id: int):
-    """Plan #693: start or stop a run-until-done backfill. Body:
-    `{"action": "start" | "stop"}` (default "start"). 'start' walks the full
-    post history in time-boxed chunks until it reaches the bottom (then the
-    source shows 'complete'); 'stop' cancels back to tick mode. Returns the
-    updated source dict (incl. backfill_state / backfill_chunks)."""
+    """Plan #693/#697: start/stop a run-until-done backfill, or start a recovery.
+    Body: `{"action": "start" | "stop" | "recover"}` (default "start"). 'start'
+    walks the full post history in time-boxed chunks until it reaches the bottom
+    (then the source shows 'complete'); 'recover' is the same walk but bypasses
+    the Patreon seen-ledger to re-fetch dropped-and-deleted near-dups under the
+    current pHash threshold; 'stop' cancels either back to tick mode. Returns the
+    updated source dict (incl. backfill_state / backfill_chunks /
+    backfill_bypass_seen)."""
     payload = await request.get_json(silent=True) or {}
     action = payload.get("action", "start")
-    if action not in ("start", "stop"):
-        return _bad("invalid_action", detail="action must be 'start' or 'stop'")
+    if action not in ("start", "stop", "recover"):
+        return _bad(
+            "invalid_action",
+            detail="action must be 'start', 'stop', or 'recover'",
+        )
     async with get_session() as session:
         try:
             svc = SourceService(session)
-            record = (
-                await svc.start_backfill(source_id)
-                if action == "start"
-                else await svc.stop_backfill(source_id)
-            )
+            if action == "start":
+                record = await svc.start_backfill(source_id)
+            elif action == "recover":
+                record = await svc.start_recovery(source_id)
+            else:
+                record = await svc.stop_backfill(source_id)
         except LookupError:
             return _bad("not_found", status=404)
     return jsonify(record.to_dict())