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/config/config.go

@@ -35,6 +35,13 @@ const (
 	// authenticated activity before it expires.
 	defaultSessionIdleTimeout = 24 * time.Hour
 
+	// defaultReceiverRateLimit is the default number of requests
+	// per minute each client IP may send to a single webhook
+	// receiver entrypoint. Generous for legitimate webhook
+	// senders while bounding abuse of the one unauthenticated,
+	// internet-exposed endpoint.
+	defaultReceiverRateLimit = 120
+
 	// maxPort is the highest valid TCP port number. The lower
 	// bound (at least 1) is enforced by envPositiveInt.
 	maxPort = 65535
@@ -79,6 +86,10 @@ type Config struct {
 	// which a session expires. Non-positive disables idle expiry.
 	SessionIdleTimeout time.Duration
 
+	// ReceiverRateLimit is the number of requests per minute each
+	// client IP may send to a single webhook receiver entrypoint.
+	ReceiverRateLimit int
+
 	params *ConfigParams
 	log    *slog.Logger
 }
@@ -263,6 +274,14 @@ func loadFromEnv() (*Config, error) {
 		return nil, err
 	}
 
+	receiverRateLimit, err := envPositiveInt(
+		"RECEIVER_RATE_LIMIT",
+		defaultReceiverRateLimit,
+	)
+	if err != nil {
+		return nil, err
+	}
+
 	return &Config{
 		DataDir:                envString("DATA_DIR"),
 		Debug:                  debug,
@@ -274,6 +293,7 @@ func loadFromEnv() (*Config, error) {
 		SentryDSN:              envString("SENTRY_DSN"),
 		RetentionSweepInterval: retentionSweepInterval,
 		SessionIdleTimeout:     sessionIdleTimeout,
+		ReceiverRateLimit:      receiverRateLimit,
 	}, nil
 }
 
@@ -314,6 +334,7 @@ func New(lc fx.Lifecycle, params ConfigParams) (*Config, error) {
 		"maintenanceMode", s.MaintenanceMode,
 		"dataDir", s.DataDir,
 		"retentionSweepInterval", s.RetentionSweepInterval.String(),
+		"receiverRateLimit", s.ReceiverRateLimit,
 		"hasSentryDSN", s.SentryDSN != "",
 		"hasMetricsAuth",
 		s.MetricsUsername != "" && s.MetricsPassword != "",