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_graduation.py

@@ -376,6 +376,9 @@ def make_router(
             models=super_draft_entry.models,
             funder=super_draft_entry.funder,
             body=super_draft_entry.body,
+            # INV-7 (§22.4a): carry forward-compat / unknown frontmatter keys
+            # through graduation rather than dropping them on the rebuild.
+            extra=dict(super_draft_entry.extra),
         )
         graduated_contents = entry_mod.serialize(graduated_entry)