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>

web/templates/pages/error.html

@@ -0,0 +1,18 @@
+{{define "content"}}
+{{with .Data}}
+<article>
+    <hgroup>
+        <h1>{{.Code}}</h1>
+        <p>{{.Message}}</p>
+    </hgroup>
+    {{if eq .Code 404}}
+        <p>Siden du leter etter finnes ikke. Den kan ha blitt flyttet eller slettet.</p>
+    {{else if eq .Code 403}}
+        <p>Du har ikke tilgang til denne siden.</p>
+    {{else}}
+        <p>Noe gikk galt. Prøv igjen senere.</p>
+    {{end}}
+    <p><a href="{{basePath}}/">Tilbake til forsiden</a></p>
+</article>
+{{end}}
+{{end}}