feat(subscribestar): flip dispatch to the native ingester (#893, Step 5)

SubscribeStar now downloads + verifies through the native core ingester instead
of gallery-dl — the go-live switch for milestone #71.

- download_backends: subscribestar added to NATIVE_INGESTER_PLATFORMS; a
  _NATIVE_INGESTERS registry + _resolve_native_campaign_id make _run_native_ingester
  / preview_source / verify_source_credential platform-aware. SubscribeStar's
  campaign_id IS the creator URL (no resolver); Patreon still resolves the vanity.
  preview now catches the shared NativeIngestError (covers both platforms).
- platform_lock: subscribestar serialized (one paced walk at a time).
- gallery_dl: subscribestar entry removed from PLATFORM_DEFAULTS (rule 22 — no
  fallback once native works).
- frontend SourceActions: isPatreon → isNative (patreon|subscribestar) so the
  recover/recapture actions show for subscribestar; download_service's
  cursor/mode/post_first + the preview endpoint already key on
  uses_native_ingester, so backfill/recovery/recapture/preview light up for free.
- tests: download_backends (subscribestar native), platform_lock (serialized),
  and three gallery-dl-sample tests repointed to hentaifoundry (api_credentials
  verify, gallery_dl_service skip-value, api_sources arm-no-preflight).

post_is_gated stays best-effort (can't cause junk downloads); not gating this.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/components/subscriptions/SourceActions.vue

@@ -28,7 +28,7 @@
 
     <KebabMenu size="small" :min-width="260" label="More actions">
       <v-list-item
-        v-if="isPatreon && !running"
+        v-if="isNative && !running"
         prepend-icon="mdi-backup-restore"
         @click="emit('recover', source)"
       >
@@ -39,7 +39,7 @@
         </v-list-item-subtitle>
       </v-list-item>
       <v-list-item
-        v-if="isPatreon && !running"
+        v-if="isNative && !running"
         prepend-icon="mdi-text-box-search-outline"
         @click="emit('recapture', source)"
       >
@@ -49,7 +49,7 @@
           already on disk — without re-downloading media
         </v-list-item-subtitle>
       </v-list-item>
-      <v-divider v-if="isPatreon && !running" />
+      <v-divider v-if="isNative && !running" />
       <v-list-item
         base-color="error"
         prepend-icon="mdi-delete-outline"
@@ -74,7 +74,10 @@ const emit = defineEmits(['check', 'backfill', 'recover', 'recapture', 'remove']
 const running = computed(() => props.source.backfill_state === 'running')
 const recovering = computed(() => !!props.source.backfill_bypass_seen)
 const recapturing = computed(() => !!props.source.backfill_recapture)
-const isPatreon = computed(() => props.source.platform === 'patreon')
+// Recover / recapture are native-ingester features (ledger-bypass re-walk and
+// post-text re-grab), available to every native platform — not just Patreon.
+const NATIVE_PLATFORMS = ['patreon', 'subscribestar']
+const isNative = computed(() => NATIVE_PLATFORMS.includes(props.source.platform))
 </script>
 
 <style scoped>