feat(pixiv): flip dispatch to the native ingester (#129 step 4)

pixiv joins NATIVE_INGESTER_PLATFORMS: download/verify/preview and the
recover/recapture UI actions now route through PixivIngester. Campaign id is
parsed straight from the source URL (numeric user id — no network resolver),
with a platform-aware resolution-failure message. auth_token now rides the
uniform adapter construction (token platforms use it, cookie platforms
accept-and-ignore), and the preview endpoint fetches/threads it. The legacy
gallery-dl pixiv path is fully removed (PLATFORM_DEFAULTS entry + the
refresh-token config branches in download/verify) per no-legacy policy;
gallery-dl keeps hentaifoundry/discord/deviantart until they migrate/retire.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/services/subscribestar_ingester.py

@@ -61,9 +61,14 @@ class SubscribeStarIngester(Ingester):
         validate: bool = True,
         rate_limit: float = 0.0,
         request_sleep: float = 0.0,
+        auth_token: str | None = None,
         client: SubscribeStarClient | None = None,
         downloader: SubscribeStarDownloader | None = None,
     ):
+        # auth_token: accepted for the uniform native-ingester construction
+        # (download_backends passes it to every adapter); SubscribeStar
+        # authenticates by cookies, so it's unused here.
+        del auth_token
         self.images_root = Path(images_root)
         self.cookies_path = str(cookies_path) if cookies_path else None
         resolved_client = (