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>

nfpm.yaml

@@ -0,0 +1,52 @@
+# nfpm configuration for building .deb and .rpm packages.
+# https://nfpm.goreleaser.com/
+# SPDX-License-Identifier: AGPL-3.0-or-later
+#
+# Usage:
+#   ARCH=amd64 VERSION=1.0.0 nfpm package --packager deb --target dist/
+#   ARCH=arm64 VERSION=1.0.0 nfpm package --packager rpm --target dist/
+
+name: favoritter
+arch: "${ARCH}"
+platform: linux
+version: "${VERSION}"
+maintainer: "Ole M. <olemd@kode.naiv.no>"
+description: "Self-hosted favorites web app"
+vendor: ""
+homepage: "https://kode.naiv.no/olemd/favoritter"
+license: AGPL-3.0-or-later
+
+contents:
+  - src: ./dist/favoritter_${VERSION}_linux_${ARCH}
+    dst: /usr/bin/favoritter
+    file_info:
+      mode: 0755
+  - src: ./dist/favoritter.service
+    dst: /lib/systemd/system/favoritter.service
+  - src: ./dist/favoritter.env
+    dst: /etc/favoritter/favoritter.env
+    type: config|noreplace
+  - dst: /var/lib/favoritter
+    type: dir
+    file_info:
+      mode: 0750
+      owner: favoritter
+      group: favoritter
+  - dst: /var/lib/favoritter/uploads
+    type: dir
+    file_info:
+      mode: 0750
+      owner: favoritter
+      group: favoritter
+
+scripts:
+  postinstall: ./dist/postinstall.sh
+  preremove: ./dist/preremove.sh
+
+overrides:
+  deb:
+    depends:
+      - libc6
+  rpm:
+    depends:
+      - glibc