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/api.py

@@ -651,6 +651,7 @@ def make_router(
             f"""
             SELECT r.slug, r.title, r.state, r.rfc_id, r.repo,
                    r.owners_json, r.arbiters_json, r.tags_json,
+                   r.metadata_malformed,
                    r.last_main_commit_at, r.last_entry_commit_at, r.updated_at
               FROM cached_rfcs r JOIN collections c ON c.id = r.collection_id
              WHERE r.state IN ('super-draft', 'active')
@@ -684,6 +685,7 @@ def make_router(
                     "last_active_at": r["last_main_commit_at"] or r["last_entry_commit_at"] or r["updated_at"],
                     "starred_by_me": r["slug"] in starred,
                     "has_open_prs": False,  # wired in Slice 2 when per-RFC repos exist
+                    "metadata_malformed": bool(r["metadata_malformed"]),
                 }
             )
         return {"items": items}
@@ -744,7 +746,7 @@ def make_router(
         rows = db.conn().execute(
             f"""
             SELECT slug, title, state, rfc_id, repo,
-                   owners_json, arbiters_json, tags_json,
+                   owners_json, arbiters_json, tags_json, metadata_malformed,
                    last_main_commit_at, last_entry_commit_at, updated_at
               FROM cached_rfcs
              WHERE state IN ('super-draft', 'active')
@@ -775,6 +777,7 @@ def make_router(
                 "last_active_at": r["last_main_commit_at"] or r["last_entry_commit_at"] or r["updated_at"],
                 "starred_by_me": r["slug"] in starred,
                 "has_open_prs": False,
+                "metadata_malformed": bool(r["metadata_malformed"]),
             }
             for r in rows
         ]
@@ -1342,6 +1345,7 @@ def _serialize_rfc(row) -> dict[str, Any]:
         "arbiters": json.loads(row["arbiters_json"] or "[]"),
         "tags": json.loads(row["tags_json"] or "[]"),
         "body": row["body"] or "",
+        "metadata_malformed": bool(row["metadata_malformed"]),
     }