feat(subscribestar): seen/failed ledger models + migration 0054 (#889)

Phase 1, step 1 of moving SubscribeStar off gallery-dl onto the native core
ingester (milestone: SubscribeStar native). Mirror of the Patreon ledger:
SubscribeStarSeenMedia (skip already-ingested media on routine walks; recovery
bypasses) and SubscribeStarFailedMedia (dead-letter so persistently-failing
media stops re-burning backfill chunks). Per operator decision, dedicated
per-platform tables (not a generalized shared ledger).

filehash is String(128): a CDN content hash when the URL carries one, else a
synthesized <post_id>:<filename> key. UNIQUE (source_id, filehash) upsert key.
Registered in models/__init__; migration 0054 creates both tables (down 0053).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -24,6 +24,8 @@ from .series_chapter import SeriesChapter
 from .series_page import SeriesPage
 from .series_suggestion import SeriesSuggestion
 from .source import Source
+from .subscribestar_failed_media import SubscribeStarFailedMedia
+from .subscribestar_seen_media import SubscribeStarSeenMedia
 from .tag import Tag, TagKind, image_tag
 from .tag_alias import TagAlias
 from .tag_allowlist import TagAllowlist
@@ -41,6 +43,8 @@ __all__ = [
     "Credential",
     "PatreonFailedMedia",
     "PatreonSeenMedia",
+    "SubscribeStarFailedMedia",
+    "SubscribeStarSeenMedia",
     "Post",
     "PostAttachment",
     "SeriesChapter",