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/SourceRow.vue

@@ -34,7 +34,8 @@
       <v-chip
         v-else-if="source.backfill_state === 'running'"
         size="x-small" color="info" variant="tonal" label
-      >Backfilling{{ source.backfill_chunks ? ` (${source.backfill_chunks})` : '' }}</v-chip>
+      >{{ source.backfill_bypass_seen ? 'Recovering' : 'Backfilling'
+        }}{{ source.backfill_chunks ? ` (${source.backfill_chunks})` : '' }}</v-chip>
       <v-chip
         v-else-if="source.backfill_state === 'complete'"
         size="x-small" color="success" variant="tonal" label
@@ -62,10 +63,20 @@
         <v-icon>{{ source.backfill_state === 'running' ? 'mdi-stop' : 'mdi-magnify-scan' }}</v-icon>
         <v-tooltip activator="parent" location="top">
           {{ source.backfill_state === 'running'
-            ? 'Stop backfill'
+            ? (source.backfill_bypass_seen ? 'Stop recovery' : 'Stop backfill')
             : 'Backfill — walk the full post history until complete' }}
         </v-tooltip>
       </v-btn>
+      <v-btn
+        v-if="source.platform === 'patreon' && source.backfill_state !== 'running'"
+        size="x-small" variant="text"
+        @click.stop="$emit('recover', source)"
+      >
+        <v-icon>mdi-backup-restore</v-icon>
+        <v-tooltip activator="parent" location="top">
+          Recover — re-fetch dropped near-dups &amp; re-evaluate under the current threshold
+        </v-tooltip>
+      </v-btn>
       <v-btn
         icon="mdi-pencil" size="x-small" variant="text"
         @click.stop="$emit('edit', source)"
@@ -93,7 +104,7 @@ const props = defineProps({
   checking: { type: Boolean, default: false },
   warningThreshold: { type: Number, default: 5 },
 })
-const emit = defineEmits(['edit', 'remove', 'toggle', 'check', 'backfill'])
+const emit = defineEmits(['edit', 'remove', 'toggle', 'check', 'backfill', 'recover'])
 
 function onToggleEnabled(value) {
   emit('toggle', { source: props.source, enabled: value })