feat: migrate from Docker Compose to k3s

[dddictionary]

Summary

• Add /healthz endpoint (DB health check) for k8s readiness/liveness probes
• Add k8s manifests: backend (2 replicas, rolling update), MariaDB (StatefulSet), Traefik ingress with cert-manager TLS, full monitoring stack (Prometheus, Grafana, node-exporter)
• Rewrite deploy workflow to build images in CI → push to GHCR → rolling update via kubectl set image, eliminating downtime from on-server Rust compilation

Test plan

• cargo check passes (verified locally)
• kubectl apply --dry-run=server -R -f k8s/ on VPS after k3s install
• Push to main → GitHub Actions builds image → deploys via rolling update
• curl -I https://4brar.me returns 200
• curl -I https://observe.4brar.me returns 200
• Zero-downtime: no non-200 responses during a deploy

VPS prerequisites

1. Install k3s: curl -sfL https://get.k3s.io | sh -
2. Install cert-manager: kubectl apply -f https://github.com/cert-manager/cert-manager/releases/latest/download/cert-manager.yaml