Fix two resubmit comments and test the resubmit route's middleware (closes #252)

Two comments named the wrong mechanism: loadResubmitSource credited soft-delete for refusing a reaped event, though the retention reaper deletes event rows outright, and createAndFanOut claimed to be the only path that creates deliveries, though per-delivery replay creates one without an event. Both now say what the code does. The resubmit route's middleware had no tests through the router; new tests drive the production router to pin the refusal without a valid CSRF token, the rate limit, signed-out requests never spending it, and another webhook's event refused by the event lookup while the user's own event is accepted. Each fails with its check removed.

Model: opus-5-5

internal/handlers/event_resubmit.go

@@ -145,8 +145,9 @@ func (h *Handlers) resubmitEvent(
 // per-webhook database files — a sibling webhook's event is not in the
 // database being queried at all — and is there so the scoping survives
 // any future change that puts more than one webhook's events in one
-// file. Going through Model applies GORM's soft-delete scope, which is
-// what stops a reaped event being resubmitted.
+// file. A reaped event is not found because the retention reaper
+// deletes its row outright rather than marking it deleted; see
+// deleteEvents in internal/database/retention.go.
 func loadResubmitSource(
 	webhookDB *gorm.DB,
 	webhookID, eventID string,