CUB-117: Port Moonraker + MQTT printer integrations to Go

- Moonraker REST client with GetPrinterInfo, GetPrintStats, GetPrintHistory
- Moonraker WebSocket client with auto-reconnect + telemetry parsing
- MQTT client via paho.mqtt.golang with TLS support for Bambu Lab
- Moonraker poller worker: background polling, dedup, usage logging to PostgreSQL
- MQTT subscriber worker: Bambu telemetry parsing, print job tracking
- Config: 7 new env vars (MOONRAKER_URL, MQTT_BROKER, etc.)
- main.go: per-printer worker discovery, graceful shutdown

backend/internal/config/config.go

@@ -12,6 +12,17 @@ type Config struct {
 	Port        string `envconfig:"port" default:"8080"`
 	CorsOrigin  string `envconfig:"cors_origin" default:"*"`
 	LogLevel    string `envconfig:"log_level" default:"info"`
+
+	// Moonraker integration.
+	MoonrakerURL          string `envconfig:"moonraker_url" default:"http://localhost:7125"`
+	MoonrakerPollInterval string `envconfig:"moonraker_poll_interval" default:"10s"`
+
+	// MQTT (Bambu Lab) integration.
+	MQTTBroker      string `envconfig:"mqtt_broker" default:"localhost:1883"`
+	MQTTTopicPrefix string `envconfig:"mqtt_topic_prefix" default:"device/+/report"`
+	MQTTClientID    string `envconfig:"mqtt_client_id" default:"extrudex"`
+	MQTTTLSCert     string `envconfig:"mqtt_tls_cert" default:""`
+	MQTTTLSKey      string `envconfig:"mqtt_tls_key" default:""`
 }
 
 // Load reads configuration from environment variables and returns a populated Config.