feat(vpn): VPN on-demand Tailscale + agent studioE5 standalone

- Agent studioE5 standalone en Go (console + systray)
- VPN on-demand via tailscaled + tailscale up (authkey Headscale)
- Resolver/serveur dans le tailnet studioe5
- Caddy on-demand TLS pour les instances
- Nouveaux endpoints serveur /api/internal/send-to-node
- Suppression des anciens binaires edubox-agent
- Suivi dans SUIVI_VPN_ONDEMAND.md

Caddyfile

@@ -6,7 +6,7 @@
 }
 
 :80 {
-    route /edubox-agent* {
+    route /studioE5-agent* {
         file_server {
             root /usr/share/caddy/agent
         }
@@ -22,11 +22,11 @@
     }
 }
 
-headscale.alfrednobel.edudeploy.com {
+headscale.studioe5.edudeploy.com:443 {
     reverse_proxy headscale:8080
 }
 
-alfrednobel.edudeploy.com {
+studioe5.edudeploy.com:443 {
     reverse_proxy /api/websocket* server:3001
     reverse_proxy server:3000
 }
@@ -36,9 +36,9 @@ alfrednobel.edudeploy.com {
         on_demand
     }
     @instance {
-        not host alfrednobel.edudeploy.com
-        not host headscale.alfrednobel.edudeploy.com
-        host *.alfrednobel.edudeploy.com
+        not host studioe5.edudeploy.com
+        not host headscale.studioe5.edudeploy.com
+        host *.studioe5.edudeploy.com
     }
     handle @instance {
         reverse_proxy resolver:2020 {