feat(server): handlers REST + transport MQTT

Ajout des handlers HTTP (agents, métriques historique, config agent/serveur, icônes upload/get) et du client MQTT serveur avec subscribe automatique et PushConfig.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/transport/mqtt.go

@@ -0,0 +1,61 @@
+package transport
+
+import (
+	"encoding/json"
+	"fmt"
+	"log"
+
+	mqtt "github.com/eclipse/paho.mqtt.golang"
+	"github.com/user/nanometrics/server/models"
+)
+
+type MQTTClient struct {
+	client    mqtt.Client
+	topicBase string
+}
+
+func StartMQTT(broker, topicBase string, handler func(*models.AgentMetrics)) (*MQTTClient, error) {
+	mc := &MQTTClient{topicBase: topicBase}
+
+	opts := mqtt.NewClientOptions().
+		AddBroker(broker).
+		SetClientID("nanometrics-server").
+		SetAutoReconnect(true).
+		SetOnConnectHandler(func(c mqtt.Client) {
+			log.Printf("[mqtt] connecté à %s", broker)
+			topic := fmt.Sprintf("%s/+/metrics", topicBase)
+			if tok := c.Subscribe(topic, 0, nil); tok.Wait() && tok.Error() != nil {
+				log.Printf("[mqtt] subscribe error: %v", tok.Error())
+			}
+		}).
+		SetConnectionLostHandler(func(c mqtt.Client, err error) {
+			log.Printf("[mqtt] connexion perdue: %v", err)
+		}).
+		SetDefaultPublishHandler(func(_ mqtt.Client, msg mqtt.Message) {
+			var m models.AgentMetrics
+			if err := json.Unmarshal(msg.Payload(), &m); err != nil {
+				log.Printf("[mqtt] JSON invalide sur %s: %v", msg.Topic(), err)
+				return
+			}
+			if m.Hostname != "" {
+				handler(&m)
+			}
+		})
+
+	mc.client = mqtt.NewClient(opts)
+	if tok := mc.client.Connect(); tok.Wait() && tok.Error() != nil {
+		return nil, fmt.Errorf("mqtt connect: %w", tok.Error())
+	}
+	return mc, nil
+}
+
+func (mc *MQTTClient) PushConfig(hostname string, cfg *models.AgentConfig) error {
+	topic := fmt.Sprintf("%s/%s/config", mc.topicBase, hostname)
+	data, err := json.Marshal(cfg)
+	if err != nil {
+		return err
+	}
+	tok := mc.client.Publish(topic, 1, false, data)
+	tok.Wait()
+	return tok.Error()
+}