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/subscribestar_seen_media.py

@@ -0,0 +1,40 @@
+"""SubscribeStarSeenMedia — per-source ledger of SubscribeStar media already
+downloaded+processed.
+
+Mirror of PatreonSeenMedia for the SubscribeStar native ingester (replacing
+gallery-dl). One queryable row per (source, media) so routine walks skip media
+we've already ingested; recovery mode bypasses the ledger to re-walk.
+
+`filehash` is a CDN content hash when the media URL carries one, else a
+synthesized ``<post_id>:<filename>`` key (SubscribeStar URLs aren't always
+content-addressed) — hence String(128) rather than 32.
+"""
+
+from datetime import datetime
+
+from sqlalchemy import ForeignKey, Integer, String, UniqueConstraint, func
+from sqlalchemy.orm import Mapped, mapped_column
+from sqlalchemy.types import DateTime
+
+from .base import Base
+
+
+class SubscribeStarSeenMedia(Base):
+    __tablename__ = "subscribestar_seen_media"
+    __table_args__ = (
+        # Dedup key the downloader upserts against: one ledger row per
+        # (source, media). A second sighting of the same media is a no-op.
+        UniqueConstraint(
+            "source_id", "filehash", name="uq_subscribestar_seen_media_source_id"
+        ),
+    )
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    source_id: Mapped[int] = mapped_column(
+        ForeignKey("source.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    filehash: Mapped[str] = mapped_column(String(128), nullable=False)
+    post_id: Mapped[str | None] = mapped_column(String(64), nullable=True)
+    seen_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )