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>

backend/app/services/platform_lock.py

@@ -21,9 +21,10 @@ from ..config import get_config
 log = logging.getLogger(__name__)
 
 # Platforms walked one-at-a-time. gallery-dl platforms are intentionally NOT
-# here: each runs as a self-pacing subprocess and they're lower-volume. Add a
-# platform here to cap it to a single concurrent walk.
-SERIALIZED_PLATFORMS = frozenset({"patreon"})
+# here: each runs as a self-pacing subprocess and they're lower-volume. The
+# native-ingester platforms are serialized (one paced scrape/API walk at a time).
+# Add a platform here to cap it to a single concurrent walk.
+SERIALIZED_PLATFORMS = frozenset({"patreon", "subscribestar"})
 
 _LOCK_PREFIX = "fc:download_lock:"