Add infrastructure layer with following components:

**Reverse Proxy & SSL:**
- Caddy: Auto HTTPS with Let's Encrypt, simple configuration
- Caddyfile with reverse proxy rules for Nextcloud and Grafana

**Monitoring Stack (Observability):**
- Prometheus: Metrics collection and time-series database
- Grafana: Visualization dashboards with datasource provisioning
- Loki: Lightweight log aggregation
- Promtail: Log collection agent for Docker containers
- cAdvisor: Container resource monitoring

**Automation:**
- Watchtower: Automatic Docker image updates (label-based)
- Duplicati: Remote backup with web UI and encryption support

**Security:**
- Fail2ban: Intrusion prevention and IP banning

**Key Features:**
- All services use official Alpine-based images (lightweight)
- Network isolation (automa-proxy, automa-monitoring)
- Resource limits and health checks configured
- Read-only configs where applicable
- Comprehensive README with setup instructions

**Resource Usage:**
- Total additional overhead: ~1.5GB RAM, ~16GB disk
- Follows KISS principles and Unix philosophy
- All services replaceable and independently scalable

Refs: #3