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>

backend/app/cache.py

@@ -219,6 +219,19 @@ async def refresh_rfc_repo(config: Config, gitea: Gitea, slug: str) -> None:
         open_pulls, closed_pulls = [], []
     for pull in open_pulls + closed_pulls:
         head_branch = pull.get("head", {}).get("ref", "")
+        # Same deleted-branch recovery as refresh_meta_pulls: a merged-and-
+        # deleted PR's `head.ref` collapses to `refs/pull/<N>/head`. Here
+        # the slug is known (param), so state still updates correctly and
+        # no ghost forms — but blindly storing the sentinel would clobber
+        # the real branch name api_prs.py relies on as a fallback ref when
+        # the merge commit is gone. Recover it from the stored row.
+        if not head_branch or head_branch.startswith("refs/pull/"):
+            prior = db.conn().execute(
+                "SELECT head_branch FROM cached_prs WHERE repo = ? AND pr_number = ?",
+                (repo_full, pull["number"]),
+            ).fetchone()
+            if prior and prior["head_branch"]:
+                head_branch = prior["head_branch"]
         state = _state_from_pull(pull)
         gitea_opener = (pull.get("user") or {}).get("login") or ""
         opened_by = _resolve_actor(
@@ -431,6 +444,25 @@ async def refresh_meta_pulls(config: Config, gitea: Gitea) -> None:
 
     for pull in open_pulls + closed_pulls:
         head_branch = pull.get("head", {}).get("ref", "")
+        # A merged-and-deleted PR's branch is no longer reported by Gitea
+        # as its real name — the `head.ref` collapses to the synthetic
+        # `refs/pull/<N>/head` sentinel (or empty). The slug + kind both
+        # derive from the branch name, so a deleted branch would parse to
+        # slug=None and the row would be skipped forever, freezing the
+        # cached_prs row at its last-seen `state='open'` — a permanent
+        # ghost "pending idea" for an entry that has actually merged
+        # (caught when the operator authoring lane in ROADMAP #35 merged
+        # an idea PR with the branch deleted; the web UX leaves branches
+        # in place so it never tripped this). Recover the original branch
+        # from the row we already stored when the PR was open — that row
+        # retains the real `head_branch` (migration 002).
+        if not head_branch or head_branch.startswith("refs/pull/"):
+            prior = db.conn().execute(
+                "SELECT head_branch FROM cached_prs WHERE repo = ? AND pr_number = ?",
+                (repo_full, pull["number"]),
+            ).fetchone()
+            if prior and prior["head_branch"]:
+                head_branch = prior["head_branch"]
         slug = _slug_from_head_branch(head_branch)
         if slug is None:
             continue