§13: make graduation's RFC number optional; add retire (soft delete)

Optional number (§13.2/§13.3): GraduateBody.rfc_id is now optional.
A blank/absent id graduates to `active` with `id: null`, leaving the
slug as the canonical identifier (§2.3). /graduate/check treats a blank
id as valid (ok:true); /graduate only validates the RFC-NNNN regex +
collision check when a number is supplied. The Graduate dialog allows an
empty number and renders number-less entries by slug (no RFC-undefined).

Retire (§3, §3.1, §13.7): new `retired` soft-delete state. An RFC's own
owners (frontmatter) and site `owner`-role holders — not app admins —
retire via POST /api/rfcs/<slug>/retire, an auto-merged meta-repo flip
PR (graduation machinery reused). Retired entries leave every browsing
surface: catalog, get_rfc (404 except site owner), discussion/branch
reads. Un-retire is site-owner-only (POST .../unretire), discoverable
via the owner-gated GET /api/admin/retired-rfcs ("Retired" admin tab).
Migration 025 widens the cached_rfcs.state CHECK to include 'retired'
(table rebuild, all columns preserved).

Tests: graduation no-number cases (active+null id by slug; check accepts
blank) added to test_graduation_vertical.py; new test_retire_vertical.py
covers perms (owner/site-owner allowed, admin/contributor 403),
catalog/read exclusion, and a graduate→retire→unretire round-trip. Full
backend suite 386 passing; frontend builds clean. SPEC §3/§3.1/§13
updated; CHANGELOG + VERSION → 0.33.0.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "rfc-app-frontend",
   "private": true,
-  "version": "0.31.4",
+  "version": "0.32.0",
   "type": "module",
   "scripts": {
     "dev": "vite",