feat: implement queue pruning and message rotation

Enforce QUEUE_MAX_AGE and MAX_HISTORY config values that previously
existed but were not applied. The existing cleanup loop now also:

- Prunes client_queues entries older than QUEUE_MAX_AGE (default 48h)
- Rotates messages per target (channel/DM) beyond MAX_HISTORY (default 10000)
- Removes orphaned messages no longer referenced by any client queue

closes #40

internal/config/config.go

@@ -40,6 +40,7 @@ type Config struct {
 	SentryDSN          string
 	MaxHistory         int
 	MaxMessageSize     int
+	QueueMaxAge        int
 	MOTD               string
 	ServerName         string
 	FederationKey      string
@@ -70,6 +71,7 @@ func New(
 	viper.SetDefault("METRICS_PASSWORD", "")
 	viper.SetDefault("MAX_HISTORY", "10000")
 	viper.SetDefault("MAX_MESSAGE_SIZE", "4096")
+	viper.SetDefault("QUEUE_MAX_AGE", "172800")
 	viper.SetDefault("MOTD", defaultMOTD)
 	viper.SetDefault("SERVER_NAME", "")
 	viper.SetDefault("FEDERATION_KEY", "")
@@ -94,6 +96,7 @@ func New(
 		MetricsPassword:    viper.GetString("METRICS_PASSWORD"),
 		MaxHistory:         viper.GetInt("MAX_HISTORY"),
 		MaxMessageSize:     viper.GetInt("MAX_MESSAGE_SIZE"),
+		QueueMaxAge:        viper.GetInt("QUEUE_MAX_AGE"),
 		MOTD:               viper.GetString("MOTD"),
 		ServerName:         viper.GetString("SERVER_NAME"),
 		FederationKey:      viper.GetString("FEDERATION_KEY"),