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/moonraker.go

@@ -0,0 +1,161 @@
+// Package clients provides third-party printer integrations.
+package clients
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"log/slog"
+	"net/http"
+	"time"
+)
+
+// MoonrakerPrinterInfo represents the response from /api/printer/info.
+type MoonrakerPrinterInfo struct {
+	State      string `json:"state"`
+	Hostname   string `json:"hostname,omitempty"`
+	SoftwareVersion string `json:"software_version,omitempty"`
+}
+
+// MoonrakerPrintStats represents the response from /api/printer/print_stats.
+type MoonrakerPrintStats struct {
+	State            string  `json:"state"`
+	Filename         string  `json:"filename,omitempty"`
+	FilamentUsedMm   float64 `json:"filament_used,omitempty"`
+	TotalDuration    float64 `json:"total_duration,omitempty"`
+	PrintDuration    float64 `json:"print_duration,omitempty"`
+	Message          string  `json:"message,omitempty"`
+}
+
+// MoonrakerPrintJob represents a single job from the history API.
+type MoonrakerPrintJob struct {
+	JobID          string    `json:"job_id,omitempty"`
+	Filename       string    `json:"filename"`
+	Status         string    `json:"status"`
+	StartTime      time.Time `json:"start_time"`
+	EndTime        time.Time `json:"end_time,omitempty"`
+	FilamentUsedMm float64   `json:"filament_used,omitempty"`
+	TotalDuration  float64   `json:"total_duration,omitempty"`
+}
+
+// MoonrakerHistoryResponse represents the response from /api/server/history/job.
+type MoonrakerHistoryResponse struct {
+	Items []MoonrakerPrintJob `json:"jobs"`
+}
+
+// MoonrakerClient is an HTTP client for the Moonraker API.
+type MoonrakerClient struct {
+	HTTPClient *http.Client
+}
+
+// NewMoonrakerClient creates a MoonrakerClient with the given request timeout.
+func NewMoonrakerClient(timeout time.Duration) *MoonrakerClient {
+	return &MoonrakerClient{
+		HTTPClient: &http.Client{Timeout: timeout},
+	}
+}
+
+// baseURL builds the Moonraker base URL from host and port.
+func (c *MoonrakerClient) baseURL(host string, port int) string {
+	if port == 0 {
+		port = 80
+	}
+	return fmt.Sprintf("http://%s:%d", host, port)
+}
+
+// GetPrinterInfo fetches printer info from Moonraker.
+func (c *MoonrakerClient) GetPrinterInfo(ctx context.Context, host string, port int, apiKey string) (*MoonrakerPrinterInfo, error) {
+	url := c.baseURL(host, port) + "/api/printer/info"
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return nil, err
+	}
+	if apiKey != "" {
+		req.Header.Set("X-Api-Key", apiKey)
+	}
+
+	resp, err := c.HTTPClient.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("moonraker getPrinterInfo request failed: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("moonraker getPrinterInfo returned status %d", resp.StatusCode)
+	}
+
+	var body struct {
+		Result MoonrakerPrinterInfo `json:"result"`
+	}
+	if err := json.NewDecoder(resp.Body).Decode(&body); err != nil {
+		return nil, fmt.Errorf("moonraker getPrinterInfo decode failed: %w", err)
+	}
+
+	slog.Debug("moonraker printer info", "host", host, "state", body.Result.State)
+	return &body.Result, nil
+}
+
+// GetPrintStats fetches current print statistics from Moonraker.
+func (c *MoonrakerClient) GetPrintStats(ctx context.Context, host string, port int, apiKey string) (*MoonrakerPrintStats, error) {
+	url := c.baseURL(host, port) + "/api/printer/print_stats"
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return nil, err
+	}
+	if apiKey != "" {
+		req.Header.Set("X-Api-Key", apiKey)
+	}
+
+	resp, err := c.HTTPClient.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("moonraker getPrintStats request failed: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("moonraker getPrintStats returned status %d", resp.StatusCode)
+	}
+
+	var body struct {
+		Result MoonrakerPrintStats `json:"result"`
+	}
+	if err := json.NewDecoder(resp.Body).Decode(&body); err != nil {
+		return nil, fmt.Errorf("moonraker getPrintStats decode failed: %w", err)
+	}
+
+	slog.Debug("moonraker print stats", "host", host, "state", body.Result.State, "filename", body.Result.Filename)
+	return &body.Result, nil
+}
+
+// GetPrintHistory fetches completed print job history from Moonraker.
+func (c *MoonrakerClient) GetPrintHistory(ctx context.Context, host string, port int, apiKey string, limit int) (*MoonrakerHistoryResponse, error) {
+	if limit <= 0 {
+		limit = 25
+	}
+	url := fmt.Sprintf("%s/api/server/history/job?limit=%d", c.baseURL(host, port), limit)
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return nil, err
+	}
+	if apiKey != "" {
+		req.Header.Set("X-Api-Key", apiKey)
+	}
+
+	resp, err := c.HTTPClient.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("moonraker getPrintHistory request failed: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("moonraker getPrintHistory returned status %d", resp.StatusCode)
+	}
+
+	var body MoonrakerHistoryResponse
+	if err := json.NewDecoder(resp.Body).Decode(&body); err != nil {
+		return nil, fmt.Errorf("moonraker getPrintHistory decode failed: %w", err)
+	}
+
+	slog.Debug("moonraker print history", "host", host, "count", len(body.Items))
+	return &body, nil
+}