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>

frontend/src/components/subscriptions/SubscriptionsTab.vue

@@ -175,6 +175,7 @@
                     @toggle="toggleSourceEnabled"
                     @check="onCheck"
                     @backfill="onBackfill"
+                    @recover="onRecover"
                   />
                   <tr v-if="item.sources.length === 0">
                     <td colspan="8" class="fc-subs__sources-empty">
@@ -251,6 +252,7 @@
             @toggle="toggleSourceEnabled"
             @check="onCheck"
             @backfill="onBackfill"
+            @recover="onRecover"
           />
           <div v-if="item.sources.length === 0" class="fc-subs__sources-empty">
             No sources yet. Tap + to add one.
@@ -554,6 +556,22 @@ async function onBackfill(source) {
   }
 }
 
+// Plan #697: arm a recovery walk (Patreon-only) — re-fetches dropped-and-deleted
+// near-dups and re-evaluates them under the current pHash threshold. Reuses the
+// backfill lifecycle/badge; stop via the same Stop control (onBackfill).
+async function onRecover(source) {
+  try {
+    await store.recoverSource(source.id, source.artist_id)
+    toast({ text: `Recovery started for ${source.artist_name}`, type: 'success' })
+    await store.loadAll()
+  } catch (e) {
+    toast({
+      text: `Recovery start failed: ${e?.detail || e?.message || e}`,
+      type: 'error',
+    })
+  }
+}
+
 async function checkAll(group) {
   let ok = 0
   let conflict = 0