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>

tests/test_gallery_dl_service.py

@@ -259,7 +259,7 @@ def test_build_config_emits_tick_skip_value(gdl):
     scans once 20 contiguous archived items are seen."""
     from backend.app.services.gallery_dl import TICK_SKIP_VALUE
     cfg = gdl._build_config_for_source(
-        platform="subscribestar",
+        platform="hentaifoundry",
         source_config=SourceConfig(),
         artist_slug="alice",
         skip_value=TICK_SKIP_VALUE,
@@ -272,7 +272,7 @@ def test_build_config_emits_backfill_skip_value(gdl):
     full post history."""
     from backend.app.services.gallery_dl import BACKFILL_SKIP_VALUE
     cfg = gdl._build_config_for_source(
-        platform="subscribestar",
+        platform="hentaifoundry",
         source_config=SourceConfig(),
         artist_slug="alice",
         skip_value=BACKFILL_SKIP_VALUE,