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/handlers/agents.go

@@ -0,0 +1,20 @@
+package handlers
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/user/nanometrics/server/db"
+)
+
+func AgentsHandler(database *db.DB) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		agents, err := database.GetAgents()
+		if err != nil {
+			http.Error(w, err.Error(), 500)
+			return
+		}
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(agents)
+	}
+}