Bound the receiver rate limit per client IP across /webhook/* (closes #139)

The receiver limiter keyed on the request path, and /webhook/{uuid} matches any single segment, so a client minted a fresh bucket per invented path and had unlimited aggregate rate against the only unauthenticated endpoint. An outer limiter keyed on the client address alone now bounds that, chained in front of the unchanged per-entrypoint limiter. Its rejections log at DEBUG without the path, and the README states what each limit does and does not bound.

internal/middleware/export_test.go

@@ -41,3 +41,13 @@ const LoginRateLimitConst = loginRateLimit
 // PasswordChangeRateLimitConst exposes the
 // passwordChangeRateLimit constant.
 const PasswordChangeRateLimitConst = passwordChangeRateLimit
+
+// ReceiverAggregateMultiplierConst exposes the
+// receiverAggregateMultiplier constant.
+const ReceiverAggregateMultiplierConst = receiverAggregateMultiplier
+
+// ReceiverAggregateLimitForTest exposes receiverAggregateLimit for
+// testing.
+func ReceiverAggregateLimitForTest(perEntrypoint int) int {
+	return receiverAggregateLimit(perEntrypoint)
+}