feat(provenance): ProvenanceService.for_post (post-header payload)

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

tests/test_provenance_service.py

@@ -129,3 +129,27 @@ async def test_for_image_null_post_fields_serialize_null(db):
     assert e["post"]["title"] is None
     assert e["post"]["description_html"] is None
     assert e["post"]["attachment_count"] is None
+
+
+@pytest.mark.asyncio
+async def test_for_post_missing_returns_none(db):
+    svc = ProvenanceService(db)
+    assert await svc.for_post(999999) is None
+
+
+@pytest.mark.asyncio
+async def test_for_post_returns_post_source_artist(db):
+    artist, source, post = await _seed_post(
+        db, artist_name="Carol", slug="carol", platform="patreon",
+        ext_id="77", title="T", desc="<p>d</p>", count=3,
+    )
+    svc = ProvenanceService(db)
+    payload = await svc.for_post(post.id)
+    assert payload["post"]["id"] == post.id
+    assert payload["post"]["title"] == "T"
+    assert payload["post"]["description_html"] == "<p>d</p>"
+    assert payload["post"]["attachment_count"] == 3
+    assert payload["source"] == {"id": source.id, "platform": "patreon",
+                                 "url": source.url}
+    assert payload["artist"] == {"id": artist.id, "name": "Carol",
+                                 "slug": "carol"}