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>

compose.yaml

@@ -5,10 +5,7 @@ include:
   - util/compose.yaml
   - ai/compose.yaml
   - restic/compose.yaml
-  - netdata/compose.yaml
-  - umami/compose.yaml
-  - proxy/compose.yaml
-  - watch/compose.yaml
+  - net/compose.yaml
   - media/compose.yaml
   - dev/compose.yaml