v0.47.0 — SLICE-1: metadata sidecars — storage + dual-read + migration tool

§22.4a SLICE-1 of docs/design/2026-06-06-configurable-collection-metadata.md
(§7.2). Entry metadata can live in a per-entry `<slug>.meta.yaml` sidecar with
the `.md` kept as pure prose (INV-2). Additive and non-breaking — with no
sidecars present every corpus stays on the legacy frontmatter path,
byte-identical (N=1 unchanged).

- Dual-read (app/metadata.py `read_entry`) — sidecar-else-legacy-frontmatter,
  identical records (INV-6); unknown/forward-compat keys ride along through
  parse→serialize and migration (INV-7, `Entry.extra`). A degenerate sidecar
  (malformed/empty/slug-less) never drops the entry — slug backstopped from the
  filename stem, flagged not lost (INV-3).
- Migration tool (`metadata.migrate_collection`) — idempotent, one ChangeFiles
  commit per collection (new `gitea.change_files`). Tested as a function; its
  Owner-gated operator trigger is DEFERRED to SLICE-4 (write paths must become
  sidecar-aware first — see the design's SLICE-4 note + INV-8). No production
  trigger ships here, so no corpus is rewritten.
- Malformed flag — migration 033 adds `cached_rfcs.metadata_malformed`
  (additive); the corpus mirror derives it; catalog list + entry-detail APIs
  surface `metadata_malformed`.
- INV-7 at graduation — graduation now carries `Entry.extra` through the rebuild
  instead of dropping forward-compat keys.

Gate: backend 575 passed (28 new: test_metadata / _migration / _cache +
graduation extra-preservation). Frontend untouched. CHANGELOG 0.47.0 +
upgrade-steps; VERSION + frontend/package.json -> 0.47.0.

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

backend/app/gitea.py

@@ -193,6 +193,40 @@ class Gitea:
         resp = await self._request("PUT", f"/repos/{owner}/{repo}/contents/{path}", json=body)
         return resp.json()
 
+    async def change_files(
+        self,
+        owner: str,
+        repo: str,
+        *,
+        files: list[dict[str, Any]],
+        message: str,
+        branch: str,
+        author_name: str | None = None,
+        author_email: str | None = None,
+    ) -> dict:
+        """Create/update/delete several files in ONE commit (Gitea ChangeFiles).
+
+        Each `files` entry is `{"operation": "create"|"update"|"delete",
+        "path": str, "content": str (for create/update), "sha": str (required
+        for update/delete)}`. Plaintext `content` is base64-encoded here.
+        Backs the §22.4a frontmatter→sidecar migration's "one commit per
+        collection" (`metadata_migrate`).
+        """
+        out_files: list[dict[str, Any]] = []
+        for f in files:
+            item: dict[str, Any] = {"operation": f["operation"], "path": f["path"]}
+            if "content" in f and f["content"] is not None:
+                item["content"] = base64.b64encode(f["content"].encode("utf-8")).decode("ascii")
+            if f.get("sha"):
+                item["sha"] = f["sha"]
+            out_files.append(item)
+        body: dict[str, Any] = {"message": message, "branch": branch, "files": out_files}
+        if author_name and author_email:
+            body["author"] = {"name": author_name, "email": author_email}
+            body["committer"] = {"name": author_name, "email": author_email}
+        resp = await self._request("POST", f"/repos/{owner}/{repo}/contents", json=body)
+        return resp.json()
+
     # ----- Pull requests -----
 
     async def list_pulls(self, owner: str, repo: str, state: str = "open") -> list[dict]: