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_api_credentials.py

@@ -165,25 +165,26 @@ async def test_verify_runs_gallery_dl_and_stamps_on_success(client, db, monkeypa
         return (True, "Credentials valid — the feed authenticated.")
     monkeypatch.setattr(gdl_mod.GalleryDLService, "verify", _fake_verify)
 
+    # hentaifoundry is a gallery-dl platform (subscribestar migrated to native).
     await client.post("/api/credentials", json={
-        "platform": "subscribestar", "credential_type": "cookies", "data": _NETSCAPE,
+        "platform": "hentaifoundry", "credential_type": "cookies", "data": _NETSCAPE,
     })
     artist = Artist(name="Maewix", slug="maewix")
     db.add(artist)
     await db.flush()
     db.add(Source(
-        artist_id=artist.id, platform="subscribestar",
-        url="https://www.subscribestar.com/maewix", enabled=True, config_overrides={},
+        artist_id=artist.id, platform="hentaifoundry",
+        url="https://www.hentai-foundry.com/user/Maewix", enabled=True, config_overrides={},
     ))
     await db.commit()
 
-    resp = await client.post("/api/credentials/subscribestar/verify")
+    resp = await client.post("/api/credentials/hentaifoundry/verify")
     body = await resp.get_json()
     assert body["valid"] is True
     assert body["last_verified"] is not None
 
     # The stamp is persisted on the credential record.
-    rec = await (await client.get("/api/credentials/subscribestar")).get_json()
+    rec = await (await client.get("/api/credentials/hentaifoundry")).get_json()
     assert rec["last_verified"] is not None