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

- Add internal/clients/moonraker.go: HTTP client for Moonraker REST API
- Add internal/clients/mqtt.go: MQTT client via paho.mqtt.golang with TLS support
- Add internal/workers/moonraker_poller.go: background polling, usage logging, SSE broadcasts
- Add internal/workers/mqtt_subscriber.go: per-printer MQTT subscriber, Bambu telemetry parsing, SSE broadcasts
- Wire workers into cmd/server/main.go (Start/Stop with graceful shutdown)

Blocked at step 4: go toolchain not installed on build host

backend/cmd/server/main.go

@@ -11,8 +11,10 @@ import (
 
 	"github.com/CubeCraft-Creations/Extrudex/backend/internal/config"
 	"github.com/CubeCraft-Creations/Extrudex/backend/internal/db"
+	"github.com/CubeCraft-Creations/Extrudex/backend/internal/repositories"
 	"github.com/CubeCraft-Creations/Extrudex/backend/internal/router"
 	"github.com/CubeCraft-Creations/Extrudex/backend/internal/sse"
+	"github.com/CubeCraft-Creations/Extrudex/backend/internal/workers"
 )
 
 func main() {
@@ -40,6 +42,11 @@ func main() {
 
 	slog.Info("database connected")
 
+	// Repositories (for background workers)
+	printerRepo := repositories.NewPrinterRepository(dbPool)
+	jobRepo := repositories.NewPrintJobRepository(dbPool)
+	usageLogRepo := repositories.NewUsageLogRepository(dbPool)
+
 	// Create SSE broadcaster and start it
 	sseBC := sse.NewBroadcaster(128)
 	sseBC.Start()
@@ -47,6 +54,19 @@ func main() {
 
 	slog.Info("sse broadcaster started")
 
+	// Start background workers
+	mrCfg := workers.DefaultMoonrakerPollerConfig()
+	mrPoller := workers.NewMoonrakerPoller(mrCfg, dbPool, printerRepo, jobRepo, usageLogRepo, sseBC)
+	mrPoller.Start()
+	defer mrPoller.Stop()
+
+	mqttCfg := workers.DefaultMQTTSubscriberConfig()
+	mqttSub := workers.NewMQTTSubscriber(mqttCfg, dbPool, printerRepo, usageLogRepo, sseBC)
+	mqttSub.Start()
+	defer mqttSub.Stop()
+
+	slog.Info("background workers started")
+
 	// Create router
 	r := router.New(cfg, dbPool, sseBC)