Rate-limit the public webhook receiver endpoint (closes #64)

The receiver was the one unauthenticated, internet-facing endpoint with no
rate limit, so a misbehaving or hostile sender could flood a webhook
without bound. RECEIVER_RATE_LIMIT (default 120/min) now caps it, keyed on
client IP plus entrypoint path so one entrypoint cannot exhaust another's
budget. Over-limit requests get 429 with Retry-After.

The limiter deliberately does not reuse postRateLimit: that helper is
POST-only and keys on IP alone, whereas the receiver must count every
method. A test locks that property in.

Config parsing follows the fail-loudly idiom: a set-but-unparseable or
non-positive value aborts startup rather than falling back to the default.

Known limitation, tracked in #88: the key still trusts forwarded headers
unconditionally, so the limit is evadable by rotating X-Forwarded-For until
trusted-proxy gating lands.

internal/server/routes.go

@@ -171,7 +171,7 @@ func (s *Server) setupSourceRoutes() {
 }
 
 func (s *Server) setupWebhookRoutes() {
-	s.router.HandleFunc(
+	s.router.With(s.mw.ReceiverRateLimit()).HandleFunc(
 		"/webhook/{uuid}",
 		s.h.HandleWebhook(),
 	)