feat: create net stack consolidating proxy, netdata, watchtower, and umami

- Create net/compose.yaml with 4 services (traefik, netdata, watchtower, umami)
- Update arty.yml with NET_* environment variables
- Update compose.yaml to include net instead of individual stacks
- Update restic volume references to net_letsencrypt_data and net_netdata_config
- Copy configuration files to net/ directory (Dockerfile, dynamic/, go.d/, etc.)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

restic/compose.yaml

@@ -119,7 +119,7 @@ volumes:
     name: util_linkwarden_meili_data
     external: true
   backup_letsencrypt_data:
-    name: proxy_letsencrypt_data
+    name: net_letsencrypt_data
     external: true
   backup_util_vaultwarden_data:
     name: util_vaultwarden_data
@@ -132,7 +132,7 @@ volumes:
     name: jelly_config
     external: true
   backup_netdata_config:
-    name: netdata_config
+    name: net_netdata_config
     external: true
   backup_ai_postgres_data:
     name: ai_postgres_data