feat(pixiv): ledger models + migration 0076 + PixivIngester adapter (#129 step 3)

pixiv_seen_media / pixiv_failed_media mirror the Patreon/SubscribeStar
ledgers (keys are always synthesized <illust_id>:p<num> / <illust_id>:ugoira
— pximg URLs carry no content hash). PixivIngester wires client/downloader/
ledgers into ingest_core with drift label 'Pixiv app API' and the new
body_canary=False opt-out: caption-less pixiv artists are common, so the
zero-bodies #862 alarm would false-positive here — the client's
response-shape drift checks cover that failure class instead. auth_token
joins the uniform adapter constructor (pixiv is the first token-auth native
platform). verify_pixiv_credential = one OAuth refresh, no feed walk.

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

backend/app/models/__init__.py

@@ -23,6 +23,8 @@ from .library_audit_run import LibraryAuditRun
 from .ml_settings import MLSettings
 from .patreon_failed_media import PatreonFailedMedia
 from .patreon_seen_media import PatreonSeenMedia
+from .pixiv_failed_media import PixivFailedMedia
+from .pixiv_seen_media import PixivSeenMedia
 from .post import Post
 from .post_attachment import PostAttachment
 from .series_chapter import SeriesChapter
@@ -48,6 +50,8 @@ __all__ = [
     "Credential",
     "PatreonFailedMedia",
     "PatreonSeenMedia",
+    "PixivFailedMedia",
+    "PixivSeenMedia",
     "SubscribeStarFailedMedia",
     "SubscribeStarSeenMedia",
     "Post",