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

The public receiver /webhook/{uuid} had no rate limiting: anyone
who learns an entrypoint UUID can flood it, inflating the
per-webhook database and the delivery queue.

Add a dedicated limit scoped to the receiver route, keyed per
client IP per request path (the path contains the entrypoint
UUID), so one misbehaving sender is throttled without affecting
other senders of the same entrypoint or other entrypoints.
Requests over the limit get a 429; httprate adds the Retry-After
header per RFC 6585. IP extraction honours X-Forwarded-For,
X-Real-IP, and True-Client-IP for reverse-proxy deployments.

The limit is RECEIVER_RATE_LIMIT requests per minute, default
120, parsed with the existing envPositiveInt strict parser: a
set-but-unparseable or non-positive value aborts startup rather
than silently falling back to the default.

Also update the README env table and Rate Limiting design
section.

internal/middleware/ratelimit.go

@@ -24,6 +24,11 @@ const (
 	// passwordChangeRateInterval is the time window for the
 	// password change rate limit.
 	passwordChangeRateInterval = 1 * time.Minute
+
+	// receiverRateInterval is the time window for the webhook
+	// receiver rate limit. The configured limit is expressed in
+	// requests per minute.
+	receiverRateInterval = 1 * time.Minute
 )
 
 // LoginRateLimit returns middleware that enforces per-IP rate
@@ -105,3 +110,37 @@ func (m *Middleware) postRateLimit(
 		})
 	}
 }
+
+// ReceiverRateLimit returns middleware that rate-limits the
+// public webhook receiver endpoint per client IP per request
+// path (the path contains the entrypoint UUID, so each sender
+// is limited per entrypoint without affecting other senders or
+// other entrypoints). The limit is Config.ReceiverRateLimit
+// requests per minute. Requests over the limit receive a 429;
+// httprate adds the Retry-After header (RFC 6585). IP
+// extraction honours X-Forwarded-For, X-Real-IP, and
+// True-Client-IP headers for reverse-proxy setups.
+func (m *Middleware) ReceiverRateLimit() func(http.Handler) http.Handler {
+	return httprate.Limit(
+		m.params.Config.ReceiverRateLimit,
+		receiverRateInterval,
+		httprate.WithKeyFuncs(
+			httprate.KeyByRealIP,
+			httprate.KeyByEndpoint,
+		),
+		httprate.WithLimitHandler(http.HandlerFunc(
+			func(w http.ResponseWriter, r *http.Request) {
+				m.log.Warn(
+					"webhook receiver rate limit exceeded",
+					"path", r.URL.Path,
+				)
+				http.Error(
+					w,
+					"Too many requests. "+
+						"Please slow down.",
+					http.StatusTooManyRequests,
+				)
+			},
+		)),
+	)
+}