webhooks: the other half of the mutations record an event !239

Stacked on !238.

Added: mr.closed, mr.reviewed, mr.edited, mr.retargeted, issue.edited, issue.labeled, issue.assigned, issue.milestoned, mr.milestoned, release.deleted. mr close and issue edit also notify participants now, which they did not.

EventKinds is the published list. TestEventKindsAreRecorded reads the RecordEvent calls out of the source and fails if either side has something the other does not — a name in only one place is a subscription that silently never fires, which is the failure #112 is about. I checked the guard is load-bearing by renaming an event and watching it fail both directions.

That list also lets webhook add --events refuse a name this forge never emits. It runs before URL validation, which resolves DNS: a typo in the event list should not need a reachable host to report.

No repo.deleted, deliberately. events.repo_id and webhooks.repo_id both cascade from repos, so recording one would delete it — and every webhook that could have received it — in the same statement. It is undeliverable by construction, not an oversight; the audit log is where a deletion is visible. Documented at the call site.

Closes #112