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>

tests/test_api_sources.py

@@ -250,3 +250,28 @@ async def test_backfill_endpoint_rejects_bad_action(client, artist, db):
 async def test_backfill_endpoint_404_when_source_missing(client):
     resp = await client.post("/api/sources/999999/backfill", json={"action": "start"})
     assert resp.status_code == 404
+
+
+@pytest.mark.asyncio
+async def test_backfill_endpoint_recover_arms_bypass(client, artist, db):
+    """Plan #697: action='recover' arms a backfill that bypasses the Patreon
+    seen-ledger; the response exposes backfill_bypass_seen=True so the UI badge
+    can label it 'Recovering'. Stop clears it."""
+    src = Source(
+        artist_id=artist.id, platform="patreon",
+        url="https://patreon.com/alice-recover", enabled=True,
+    )
+    db.add(src)
+    await db.commit()
+    sid = src.id
+
+    resp = await client.post(f"/api/sources/{sid}/backfill", json={"action": "recover"})
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert body["backfill_state"] == "running"
+    assert body["backfill_bypass_seen"] is True
+
+    stopped = await client.post(f"/api/sources/{sid}/backfill", json={"action": "stop"})
+    stopped_body = await stopped.get_json()
+    assert stopped_body["backfill_state"] is None
+    assert stopped_body["backfill_bypass_seen"] is False