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_source_service.py

@@ -225,6 +225,38 @@ async def test_stop_backfill_returns_to_idle(db):
     assert "_backfill_state" not in (co or {})
 
 
+@pytest.mark.asyncio
+async def test_start_recovery_arms_bypass_flag(db):
+    """Plan #697: start_recovery arms the backfill state machine PLUS the
+    _backfill_bypass_seen flag (recovery), surfaced on the record so the UI badge
+    can label it 'Recovering'. Clears any prior checkpoint state."""
+    from backend.app.services.source_service import BACKFILL_MAX_CHUNKS
+
+    artist = await _artist(db, "Alice")
+    svc = SourceService(db)
+    rec = await svc.create(
+        artist_id=artist.id, platform="patreon",
+        url="https://patreon.com/alice",
+    )
+    updated = await svc.start_recovery(rec.id)
+    assert updated.backfill_state == "running"
+    assert updated.backfill_bypass_seen is True
+    assert updated.backfill_runs_remaining == BACKFILL_MAX_CHUNKS
+
+    co = (await db.execute(
+        select(Source.config_overrides).where(Source.id == rec.id)
+    )).scalar_one()
+    assert co.get("_backfill_bypass_seen") is True
+
+    # Stop clears the bypass flag too (shared lifecycle).
+    stopped = await svc.stop_backfill(rec.id)
+    assert stopped.backfill_bypass_seen is False
+    co2 = (await db.execute(
+        select(Source.config_overrides).where(Source.id == rec.id)
+    )).scalar_one()
+    assert "_backfill_bypass_seen" not in (co2 or {})
+
+
 @pytest.mark.asyncio
 async def test_start_backfill_raises_when_source_missing(db):
     svc = SourceService(db)