v0.30.1: fix phantom pending-idea after merge-with-branch-delete

refresh_meta_pulls / refresh_rfc_repo recover a PR's slug from its
Gitea head.ref, which collapses to the refs/pull/<N>/head sentinel
once a merged PR's branch is deleted. The slug then parsed to None,
the row was skipped, and cached_prs.state froze at 'open' — so the
entry showed as both a super-draft and a pending idea. Recover the
real branch name from the stored cached_prs row when Gitea reports an
empty or sentinel ref.

Surfaced via the ROADMAP #35 operator authoring lane (CLI merge with
--delete-branch); the web UX leaves branches in place so it never hit
this. Regression test added; full suite 375 green.

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

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "rfc-app-frontend",
   "private": true,
-  "version": "0.30.0",
+  "version": "0.30.1",
   "type": "module",
   "scripts": {
     "dev": "vite",