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>

alembic/versions/0037_patreon_seen_media.py

@@ -0,0 +1,53 @@
+"""patreon_seen_media: per-source ledger of already-ingested Patreon media
+
+Revision ID: 0037
+Revises: 0036
+Create Date: 2026-06-05
+
+Native Patreon ingester (build step 2a). Replaces gallery-dl's
+archive.sqlite3 with our own queryable table. The downloader upserts one
+row per (source, media) so routine walks skip media we've already
+processed; a future "recovery" mode bypasses the ledger to re-walk.
+
+`filehash` is a 32-hex Patreon CDN MD5, OR a video sentinel of the form
+``video:<post_id>:<media_id>`` — hence String(128). The unique
+constraint on (source_id, filehash) is the dedup upsert key.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0037"
+down_revision: Union[str, None] = "0036"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "patreon_seen_media",
+        sa.Column("id", sa.Integer, primary_key=True),
+        sa.Column(
+            "source_id",
+            sa.Integer,
+            sa.ForeignKey("source.id", ondelete="CASCADE"),
+            nullable=False,
+            index=True,
+        ),
+        sa.Column("filehash", sa.String(128), nullable=False),
+        sa.Column("post_id", sa.String(64), nullable=True),
+        sa.Column(
+            "seen_at",
+            sa.DateTime(timezone=True),
+            nullable=False,
+            server_default=sa.text("NOW()"),
+        ),
+        sa.UniqueConstraint(
+            "source_id", "filehash", name="uq_patreon_seen_media_source_id"
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("patreon_seen_media")