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

loadResubmitSource credited GORM's soft-delete scope for refusing a
reaped event; the reaper deletes the row outright. createAndFanOut
claimed to be the only path that creates deliveries; per-delivery
replay creates one too.

New tests drive the resubmit route through the production router:
CSRF refuses a missing, malformed or foreign token; another user's
webhook and another webhook's event are 404; the rate limit refuses
once spent; and signed-out requests never reach that rate limit. The
handler refuses a signed-out request with the same redirect itself,
so keeping such requests off the budget is what RequireAuth adds.

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.