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/Dockerfile

@@ -0,0 +1,11 @@
+# Dockerfile for Netdata with msmtp support for email alerts
+FROM netdata/netdata:latest
+
+# Install msmtp for sending emails
+RUN apt-get update && \
+    apt-get install -y msmtp msmtp-mta ca-certificates && \
+    apt-get clean && \
+    rm -rf /var/lib/apt/lists/*
+
+# Set proper permissions for msmtp config
+RUN chmod 600 /etc/msmtprc || true