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

The receiver route /h/{uuid} has no MaxBodySize middleware, unlike the page route groups; its 1 MB cap is enforced in the handler, which owns the response senders see. Nothing at the route said so, so a reader could take the receiver for uncapped or remove the only bound on the one unauthenticated endpoint. The route registration and the handler's body-reading function now say where the cap lives and why. A routing test sends a body of exactly 1 MB and one a byte over through the production router and pins that only the second is refused, with the handler's 413 and message.

Model: opus-5-5

internal/handlers/webhook.go

@@ -150,7 +150,9 @@ func (h *Handlers) lookupEntrypoint(
 	return entrypoint, true
 }
 
-// readWebhookBody reads and validates the request body size.
+// readWebhookBody reads and validates the request body size. This is
+// the receiver's only body cap: /h/{uuid} has no MaxBodySize
+// middleware (see Server.setupWebhookRoutes).
 func (h *Handlers) readWebhookBody(
 	w http.ResponseWriter,
 	r *http.Request,