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>

dist/favoritter.env

@@ -0,0 +1,16 @@
+# Favoritter configuration
+# See README.md for all available environment variables.
+
+FAVORITTER_DB_PATH=/var/lib/favoritter/favoritter.db
+FAVORITTER_UPLOAD_DIR=/var/lib/favoritter/uploads
+FAVORITTER_LISTEN=127.0.0.1:8080
+
+# Uncomment and set on first run to create the admin user:
+# FAVORITTER_ADMIN_USERNAME=admin
+# FAVORITTER_ADMIN_PASSWORD=changeme
+
+# Set this to your public URL for correct feeds, cookies, and OpenGraph:
+# FAVORITTER_EXTERNAL_URL=https://faves.example.com
+
+# If your reverse proxy is on another machine (WireGuard/Tailscale):
+# FAVORITTER_TRUSTED_PROXIES=100.64.0.0/10