feat(patreon): seen-ledger table + model — ingester build step 2a (plan #697)

patreon_seen_media(source_id, filehash, post_id, seen_at), UNIQUE(source_id,
filehash) — our own queryable replacement for gallery-dl's archive.sqlite3.
Routine walks skip seen media; recovery mode bypasses the ledger. filehash is
a 32-hex CDN MD5 or a video:<post>:<media> sentinel (String(128)). alembic
0037 (← 0036). Integration test covers dedup + savepoint recovery.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -14,6 +14,7 @@ from .import_settings import ImportSettings
 from .import_task import ImportTask
 from .library_audit_run import LibraryAuditRun
 from .ml_settings import MLSettings
+from .patreon_seen_media import PatreonSeenMedia
 from .post import Post
 from .post_attachment import PostAttachment
 from .series_page import SeriesPage
@@ -33,6 +34,7 @@ __all__ = [
     "BackupRun",
     "Source",
     "Credential",
+    "PatreonSeenMedia",
     "Post",
     "PostAttachment",
     "SeriesPage",