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

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

backend/app/services/provenance_service.py

@@ -65,3 +65,20 @@ class ProvenanceService:
                 for ip, post, src, art in rows
             ],
         }
+
+    async def for_post(self, post_id: int) -> dict | None:
+        stmt = (
+            select(Post, Source, Artist)
+            .join(Source, Source.id == Post.source_id)
+            .join(Artist, Artist.id == Source.artist_id)
+            .where(Post.id == post_id)
+        )
+        row = (await self.session.execute(stmt)).first()
+        if row is None:
+            return None
+        post, src, art = row
+        return {
+            "post": _post_dict(post),
+            "source": _source_dict(src),
+            "artist": _artist_dict(art),
+        }