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/api/sources.py

@@ -238,6 +238,7 @@ async def preview_source_endpoint(source_id: int):
             )
         cred = CredentialService(session, _get_crypto())
         cookies_path = await cred.get_cookies_path(rec.platform)
+        auth_token = await cred.get_token(rec.platform)
 
     # The walk + ledger reads are sync (run off the request loop); the process
     # sync engine is the same one the download task uses.
@@ -249,6 +250,7 @@ async def preview_source_endpoint(source_id: int):
         cookies_path=str(cookies_path) if cookies_path else None,
         images_root=Path("/images"),
         sync_session_factory=sync_session_factory(),
+        auth_token=auth_token,
     )
     if "error" in result:
         return _bad("preview_failed", detail=result["error"], status=409)