refactor: 30-day defaults, time-based message expiry

- Change QUEUE_MAX_AGE default from 48h to 30 days (2592000s)
- Replace count-based MAX_HISTORY with time-based MESSAGE_MAX_AGE
  (default 30 days) — messages older than the configured age are
  pruned instead of keeping a fixed count per target
- Update README to reflect new defaults and time-based expiry

internal/config/config.go

@@ -38,7 +38,7 @@ type Config struct {
 	MetricsUsername    string
 	Port               int
 	SentryDSN          string
-	MaxHistory         int
+	MessageMaxAge      int
 	MaxMessageSize     int
 	QueueMaxAge        int
 	MOTD               string
@@ -69,9 +69,9 @@ func New(
 	viper.SetDefault("SENTRY_DSN", "")
 	viper.SetDefault("METRICS_USERNAME", "")
 	viper.SetDefault("METRICS_PASSWORD", "")
-	viper.SetDefault("MAX_HISTORY", "10000")
+	viper.SetDefault("MESSAGE_MAX_AGE", "2592000")
 	viper.SetDefault("MAX_MESSAGE_SIZE", "4096")
-	viper.SetDefault("QUEUE_MAX_AGE", "172800")
+	viper.SetDefault("QUEUE_MAX_AGE", "2592000")
 	viper.SetDefault("MOTD", defaultMOTD)
 	viper.SetDefault("SERVER_NAME", "")
 	viper.SetDefault("FEDERATION_KEY", "")
@@ -94,7 +94,7 @@ func New(
 		MaintenanceMode:    viper.GetBool("MAINTENANCE_MODE"),
 		MetricsUsername:    viper.GetString("METRICS_USERNAME"),
 		MetricsPassword:    viper.GetString("METRICS_PASSWORD"),
-		MaxHistory:         viper.GetInt("MAX_HISTORY"),
+		MessageMaxAge:      viper.GetInt("MESSAGE_MAX_AGE"),
 		MaxMessageSize:     viper.GetInt("MAX_MESSAGE_SIZE"),
 		QueueMaxAge:        viper.GetInt("QUEUE_MAX_AGE"),
 		MOTD:               viper.GetString("MOTD"),