feat: add packaging, deployment, error pages, and project docs

Phase 7 — Polish:
- Error page template with styled 404/403/500 pages
- Error rendering helper on Renderer

Phase 8 — Packaging & Deployment:
- Containerfile: multi-stage build, non-root user, health check,
  OCI labels with build date and git revision
- Makefile: build, test, cross-compile, deb, rpm, container,
  tarballs, checksums targets
- nfpm.yaml: .deb and .rpm package config
- systemd service: hardened with NoNewPrivileges, ProtectSystem,
  ProtectHome, PrivateTmp, RestrictSUIDSGID
- Default environment file with commented examples
- postinstall/preremove scripts (shellcheck validated)
- compose.yaml: example Podman/Docker Compose
- Caddyfile.example: subdomain, subpath, and remote proxy configs
- CHANGELOG.md for release notes
- CLAUDE.md with architecture, conventions, and quick reference

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/render/render.go

@@ -152,6 +152,18 @@ func (r *Renderer) Page(w http.ResponseWriter, req *http.Request, name string, d
 	}
 }
 
+// Error renders an error page with the given HTTP status code.
+func (r *Renderer) Error(w http.ResponseWriter, req *http.Request, code int, message string) {
+	w.WriteHeader(code)
+	r.Page(w, req, "error", PageData{
+		Title: message,
+		Data: map[string]any{
+			"Code":    code,
+			"Message": message,
+		},
+	})
+}
+
 // Partial renders a partial template (for HTMX responses).
 func (r *Renderer) Partial(w io.Writer, name string, data any) error {
 	key := "partial:" + name