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/server/routes_test.go

@@ -444,6 +444,23 @@ func (e *testEnv) countDeliveries(
 	return count
 }
 
+// countEvents reports how many events a webhook's database holds.
+func (e *testEnv) countEvents(t *testing.T, webhookID string) int64 {
+	t.Helper()
+
+	webhookDB, err := e.dbMgr.GetDB(webhookID)
+	require.NoError(t, err)
+
+	var count int64
+
+	require.NoError(
+		t,
+		webhookDB.Model(&database.Event{}).Count(&count).Error,
+	)
+
+	return count
+}
+
 // storedHash reads the current password hash for a username.
 func (e *testEnv) storedHash(t *testing.T, username string) string {
 	t.Helper()