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>

net/msmtprc

@@ -0,0 +1,20 @@
+# MSMTP configuration for Netdata email alerts
+
+# Set default values for all accounts
+defaults
+auth           on
+tls            on
+tls_trust_file /etc/ssl/certs/ca-certificates.crt
+logfile        /var/log/msmtp.log
+
+# IONOS SMTP account
+account        ionos
+host           smtp.ionos.de
+port           465
+tls_starttls   off
+from           hi@pivoine.art
+user           hi@pivoine.art
+password       jaquoment
+
+# Set default account
+account default : ionos