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>

compose.yaml

@@ -0,0 +1,24 @@
+# Example Podman/Docker Compose configuration.
+# Usage: podman-compose up -d
+
+services:
+  favoritter:
+    build:
+      context: .
+      dockerfile: Containerfile
+      args:
+        BUILD_DATE: "${BUILD_DATE:-unknown}"
+        GIT_REVISION: "${GIT_REVISION:-unknown}"
+    ports:
+      - "8080:8080"
+    volumes:
+      - favoritter-data:/data
+    environment:
+      FAVORITTER_ADMIN_USERNAME: "${FAVORITTER_ADMIN_USERNAME:-admin}"
+      FAVORITTER_ADMIN_PASSWORD: "${FAVORITTER_ADMIN_PASSWORD:?Set FAVORITTER_ADMIN_PASSWORD}"
+      FAVORITTER_EXTERNAL_URL: "${FAVORITTER_EXTERNAL_URL:-}"
+      FAVORITTER_SITE_NAME: "${FAVORITTER_SITE_NAME:-Favoritter}"
+    restart: unless-stopped
+
+volumes:
+  favoritter-data: