feat(attachments): PostAttachment model + 0009 (table + batch counter)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_migration_0009.py

@@ -0,0 +1,37 @@
+"""FC-2d-iii: post_attachment table + import_batch.attachments column."""
+
+import pytest
+
+from backend.app.models import ImportBatch, PostAttachment
+
+pytestmark = pytest.mark.integration
+
+
+def test_post_attachment_columns():
+    cols = {c.name for c in PostAttachment.__table__.columns}
+    assert {
+        "id", "post_id", "artist_id", "sha256", "path",
+        "original_filename", "ext", "mime", "size_bytes", "captured_at",
+    } <= cols
+
+
+def test_import_batch_has_attachments_counter():
+    assert "attachments" in {c.name for c in ImportBatch.__table__.columns}
+
+
+@pytest.mark.asyncio
+async def test_post_attachment_roundtrip(db):
+    from backend.app.models import Artist
+
+    a = Artist(name="Zed", slug="zed")
+    db.add(a)
+    await db.flush()
+    att = PostAttachment(
+        post_id=None, artist_id=a.id, sha256="z" + "0" * 63,
+        path="/images/attachments/z00/z.zip", original_filename="pack.zip",
+        ext=".zip", mime="application/zip", size_bytes=123,
+    )
+    db.add(att)
+    await db.flush()
+    got = await db.get(PostAttachment, att.id)
+    assert got.original_filename == "pack.zip" and got.post_id is None