feat(slice4): Owner-gated collection migrate endpoint (§22.4a PUC-5)

POST .../collections/{cid}/migrate — now safe to ship since all write paths
are sidecar-aware. Idempotent, one commit per collection.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/api_metadata.py

@@ -84,4 +84,29 @@ def make_router(config: Config, gitea: Gitea, bot: Bot) -> APIRouter:
             "meta": metadata_mod.metadata_dict(new_entry),
         }
 
+    @router.post("/api/projects/{project_id}/collections/{collection_id}/migrate")
+    async def migrate(
+        project_id: str, collection_id: str, request: Request
+    ) -> dict[str, Any]:
+        """§22.4a PUC-5: migrate a collection's legacy-frontmatter entries to
+        clean body-only `.md` + sidecars, one commit per collection. Owner-gated
+        operator action. Safe to ship now that every entry write path is
+        sidecar-aware (SLICE-4 carried work). Idempotent."""
+        viewer = auth.current_user(request)
+        if collections_mod.project_of_collection(collection_id) != project_id:
+            raise HTTPException(404, "Collection not in project")
+        if not auth.is_collection_superuser(viewer, collection_id):
+            raise HTTPException(403, "Owner access required to migrate a collection")
+        org, repo = _content_repo()
+        subfolder = collections_mod.subfolder_of(collection_id) or ""
+        try:
+            result = await metadata_mod.migrate_collection(
+                gitea, org=org, repo=repo, subfolder=subfolder,
+                actor=viewer.as_actor())
+        except GiteaError as e:
+            raise HTTPException(502, f"Gitea: {e.detail}")
+        if result["committed"]:
+            await cache.refresh_meta_repo(config, gitea)
+        return result
+
     return router