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_download_backends.py

@@ -7,15 +7,16 @@ from backend.app.services.download_backends import (
 )
 
 
-def test_patreon_is_native():
-    assert uses_native_ingester("patreon") is True
-    assert "patreon" in NATIVE_INGESTER_PLATFORMS
+def test_native_platforms():
+    for platform in ("patreon", "subscribestar"):
+        assert uses_native_ingester(platform) is True
+        assert platform in NATIVE_INGESTER_PLATFORMS
 
 
 def test_gallery_dl_platforms_are_not_native():
-    # The five platforms still served by gallery-dl must NOT route to the
-    # native ingester — guards an accidental over-broad migration.
-    for platform in ("subscribestar", "hentaifoundry", "discord", "pixiv", "deviantart"):
+    # The platforms still served by gallery-dl must NOT route to the native
+    # ingester — guards an accidental over-broad migration.
+    for platform in ("hentaifoundry", "discord", "pixiv", "deviantart"):
         assert uses_native_ingester(platform) is False