Say at the receiver route where its 1 MB body cap lives (closes #173)

/h/{uuid} has no MaxBodySize middleware, unlike the page route
groups; its cap is in the handler, which owns the 413 senders get.
A comment at the route registration now says so, and the handler's
read function notes it is the receiver's only body cap.

A new routing test sends a body exactly at the cap and one byte over
it through the production router, and checks the second is refused
with the handler's 413 and message.

Model: opus-5-5

internal/server/routes.go

@@ -304,6 +304,12 @@ func (s *Server) setupSourceRoutes() {
 }
 
 func (s *Server) setupWebhookRoutes() {
+	// No MaxBodySize here, unlike the page groups. The receiver's 1 MB
+	// body cap is in Handlers.readWebhookBody, because the handler
+	// owns the response a sender gets for an oversized body and
+	// MaxBodySize would change it. That cap is the only bound on this
+	// unauthenticated endpoint's body; TestReceiver_OversizeBodyRefused
+	// pins it.
 	s.router.With(s.mw.ReceiverRateLimit()).HandleFunc(
 		"/h/{uuid}",
 		s.h.HandleWebhook(),