Slice 8 WIP: §12 hygiene + §10.7 + routing + rollback cleanup

- Add §12 30/90 hygiene scheduler in hygiene.py, mirroring the
  DigestScheduler shape; wires next to digest in main.py with the
  same start/stop/run_tick test seam.
- Extend bot.delete_branch to accept actor=None for system gestures,
  per §15.9 (actor_user_id=NULL, on_behalf_of=bot_login).
- Convert every branches/{branch} route in api_branches.py and
  api_prs.py to {branch:path}; move the bare GET to the bottom of
  the router so deeper GETs match before greedy-path swallow.
- Extend api_prs.py's _require_pr to accept pr_kind='meta_metadata'
  so the §9.5 metadata-pane PRs land an in-app merge.
- Graduation rollback now deletes the graduate-<slug>-<6hex> branch
  after closing the PR — §19.2 candidate that lands here.
- Email-bounce webhook gains a WEBHOOK_EMAIL_BOUNCE_SECRET seam.
- FakeGitea grows a DELETE /branches/{branch:path} handler and a
  slashed-branch read; integration tests for the hygiene vertical
  cover the 30d close, 90d delete, post-merge delete, pinned
  exemption, per-user cursor preservation, no-notification rule,
  and the graduation-rollback cleanup.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/api_graduation.py

@@ -846,6 +846,25 @@ async def _undo_open_pr(*, config, gitea, bot, actor, state) -> str:
         head_branch=state.graduation_branch or "",
         slug=state.slug, reason="graduation rollback",
     )
+    # Per the §19.2 "graduation rollback's branch cleanup" candidate
+    # that Slice 8 settles: delete the dash-suffixed branch on rollback
+    # so failed-graduation branches don't accumulate on the meta repo.
+    # The §12 hygiene sweep would catch this eventually, but closing
+    # the loop here removes the chance of pile-up across retries.
+    branch_name = state.graduation_branch or ""
+    if branch_name:
+        try:
+            await bot.delete_branch(
+                actor,
+                owner=config.gitea_org,
+                repo=config.meta_repo,
+                branch=branch_name,
+                slug=state.slug,
+                action_kind="delete_post_merge_branch",
+                reason="graduation rollback",
+            )
+        except Exception:
+            log.exception("rollback: delete_branch failed for %s", branch_name)
     return f"Closed PR #{state.new_pr_number}"