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,

internal/handlers/webhook.go

@@ -272,10 +272,12 @@ func requestEventSource(
 
 // createAndFanOut writes the event and one pending delivery per target,
 // and adds them to the webhook's running totals, in a single
-// transaction, then hands the tasks to the delivery engine. It is the
-// only path by which an event and its deliveries are created, so a
-// resubmitted event is retried, SSRF-guarded and circuit-broken
-// exactly as a received one is.
+// transaction, then hands the tasks to the delivery engine. Every
+// event is created here, received or resubmitted, so a resubmitted
+// event is retried, SSRF-guarded and circuit-broken exactly as a
+// received one is. Per-delivery replay is the one other path that
+// creates a delivery: it adds one to an existing event without
+// coming through here.
 //
 // The tasks are returned as well as queued, so a caller can report how
 // many targets the event went to.