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/internal/clients/mqtt.go

@@ -0,0 +1,119 @@
+// Package clients provides third-party printer integrations.
+package clients
+
+import (
+	"context"
+	"crypto/tls"
+	"encoding/json"
+	"fmt"
+	"log/slog"
+	"time"
+
+	mqtt "github.com/eclipse/paho.mqtt.golang"
+)
+
+// MQTTClient wraps the Eclipse Paho MQTT client for printer telemetry.
+type MQTTClient struct {
+	client mqtt.Client
+}
+
+// MQTTConfig holds per-printer MQTT connection settings.
+type MQTTConfig struct {
+	BrokerHost  string
+	BrokerPort  int
+	TopicPrefix string
+	TLSEnabled  bool
+	ClientID    string
+}
+
+// BambuPrintStatus is the known Bambu Lab print-status payload shape.
+type BambuPrintStatus struct {
+	Print struct {
+		GcodeFile       string  `json:"gcode_file,omitempty"`
+		Stage           int     `json:"stage,omitempty"`
+		SubTaskName     string  `json:"subtask_name,omitempty"`
+		PrintType       string  `json:"print_type,omitempty"`
+		FilamentUsedMm  float64 `json:"mc_percent,omitempty"` // placeholder; real telemetry varies
+	} `json:"print,omitempty"`
+}
+
+// NewMQTTClient creates an MQTT client connected to the given broker.
+func NewMQTTClient(cfg MQTTConfig) (*MQTTClient, error) {
+	if cfg.BrokerPort == 0 {
+		if cfg.TLSEnabled {
+			cfg.BrokerPort = 8883
+		} else {
+			cfg.BrokerPort = 1883
+		}
+	}
+	if cfg.ClientID == "" {
+		cfg.ClientID = fmt.Sprintf("extrudex-%d", time.Now().Unix())
+	}
+
+	opts := mqtt.NewClientOptions().
+		AddBroker(fmt.Sprintf("tcp://%s:%d", cfg.BrokerHost, cfg.BrokerPort)).
+		SetClientID(cfg.ClientID).
+		SetAutoReconnect(true).
+		SetConnectTimeout(10 * time.Second).
+		SetOrderMatters(false)
+
+	if cfg.TLSEnabled {
+		opts = opts.SetTLSConfig(&tls.Config{InsecureSkipVerify: false})
+	}
+
+	client := mqtt.NewClient(opts)
+	token := client.Connect()
+	if token.Wait() && token.Error() != nil {
+		return nil, fmt.Errorf("mqtt connect failed: %w", token.Error())
+	}
+
+	slog.Info("mqtt client connected", "broker", cfg.BrokerHost, "port", cfg.BrokerPort, "tls", cfg.TLSEnabled)
+	return &MQTTClient{client: client}, nil
+}
+
+// Subscribe registers a callback for messages matching topic.
+func (c *MQTTClient) Subscribe(topic string, qos byte, callback func([]byte)) error {
+	token := c.client.Subscribe(topic, qos, func(_ mqtt.Client, msg mqtt.Message) {
+		callback(msg.Payload())
+	})
+	if token.Wait() && token.Error() != nil {
+		return fmt.Errorf("mqtt subscribe failed: %w", token.Error())
+	}
+	slog.Info("mqtt subscribed", "topic", topic, "qos", qos)
+	return nil
+}
+
+// Unsubscribe removes a subscription.
+func (c *MQTTClient) Unsubscribe(topics ...string) error {
+	token := c.client.Unsubscribe(topics...)
+	if token.Wait() && token.Error() != nil {
+		return fmt.Errorf("mqtt unsubscribe failed: %w", token.Error())
+	}
+	return nil
+}
+
+// Disconnect cleanly disconnects the MQTT client.
+func (c *MQTTClient) Disconnect(quiesceMs uint) {
+	c.client.Disconnect(quiesceMs)
+}
+
+// IsConnected returns whether the underlying client is connected.
+func (c *MQTTClient) IsConnected() bool {
+	return c.client.IsConnected()
+}
+
+// ParseBambuTelemetry attempts to parse a Bambu Lab telemetry JSON payload.
+func ParseBambuTelemetry(payload []byte) (*BambuPrintStatus, error) {
+	var msg BambuPrintStatus
+	if err := json.Unmarshal(payload, &msg); err != nil {
+		return nil, fmt.Errorf("parse bambu telemetry failed: %w", err)
+	}
+	return &msg, nil
+}
+
+// DefaultBambuTopics returns the default topic patterns for Bambu Lab printers.
+func DefaultBambuTopics(topicPrefix string) []string {
+	return []string{
+		topicPrefix + "/report",
+	}
+}