feat(slice5): POST .../meta/bulk one-commit bulk metadata edit (§22.4a PUC-2)

set/add/remove ops reusing the SLICE-4 sidecar write-through; per-entry
partial-rejection; contributor+ gated (INV-4); validated at the write
boundary. Tests cover one-commit, add/remove, partial reject, authz,
and op/field/empty guards.

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

backend/tests/test_propose_vertical.py

@@ -54,6 +54,9 @@ class FakeGitea:
         self.repos: set[tuple[str, str]] = set()
         self._pr_counter = 0
         self._commit_counter = 0
+        # count of batch ChangeFiles commits (one per /contents POST with a
+        # files[] array) — lets tests assert "N files, one commit" (§22.4a D7).
+        self.change_files_calls = 0
         self._seed_repo("wiggleverse", "meta")
         # §22 M3: the deployment's project registry. Startup refresh_registry
         # reads projects.yaml here; the single 'default' project's content_repo
@@ -288,6 +291,7 @@ class FakeGitea:
         if method == "POST" and m_batch:
             owner, repo = m_batch.groups()
             branch = payload["branch"]
+            self.change_files_calls += 1
             sha = self._next_sha()
             for f in payload["files"]:
                 op = f["operation"]