config PRs: remove the hive's config-PR webhook, poll and core merge

An operator's merge on the forge deploys a config PR through
swarm-controller's DeployRequest{rev}. The hive-side path that queued a
MergeConfigPr approval and merged the PR as `core` goes:

- the `/webhook/config-pr` receiver, its HMAC secret, the WebhookRegister
  boot node and the org-hook registration; the hive vhost's `/webhook/`
  location
- the 5-minute config-PR poll
- ApprovalKind::MergeConfigPr, its dashboard card, and the deploy DAG it
  drove (DeployWindow, MergeVerify, DeployApply, FinalizeDeploy,
  DeployTail), with verify_commit, the two-phase meta deploy, rollback
  refs, the PR-failure comment and forge/pr_merge.rs
- `fetched_sha`, `sha_short`/`pr_number` on approval events, and
  `sha`/`tag` on HelperEvent::ApprovalResolved: only the merge path set
  them

`config_repo`, `merged_pr_for_commit` and `post_pr_comment` move to
forge/pr_comment.rs for the merged-rev deploy's refusal comment.
Approvals v5 drops stored `merge_config_pr` rows; a test reopens a v4
database holding them.

Closes #4850

hive-forge/src/client.rs

@@ -23,7 +23,7 @@ const DEFAULT_URL: &str = "http://localhost:3000";
 /// Bound on reaching the forge.
 const HTTP_CONNECT_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(5);
 /// Whole-request budget for a small JSON call (an `/api/v1` GET, or the
-/// run-view log streamer's snapshot POST) — `config_pr_poll`'s forge budget.
+/// run-view log streamer's snapshot POST).
 const JSON_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(15);
 /// Whole-request budget for a GET that downloads a file body — an
 /// attachment, an Actions artifact zip, or a persisted job log. Sized