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/platforms/pixiv.py

@@ -1,8 +1,14 @@
 """Pixiv — one quirk.
 
-post_url: gallery-dl's `url` is the image URL on `i.pximg.net`. The
-post permalink follows /artworks/<id>. external_post_id (= `id`) was
-already correct, so no override there.
+post_url: the sidecar's `url` (legacy gallery-dl era) is the image URL
+on `i.pximg.net`, and the native post record (#129) writes no url key
+at all — the permalink is synthesized from `id` here either way:
+/artworks/<id>. external_post_id (= `id`) was already correct, so no
+override there.
+
+Downloads run through the native ingester (pixiv_ingester.py), not
+gallery-dl; this registry entry still owns URL validation, sidecar
+parsing, and the credential surface (the OAuth refresh token).
 """
 
 from .base import GD_DEFAULTS, PlatformInfo, str_id_value