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>

Containerfile

@@ -0,0 +1,42 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# Build: BUILDAH_FORMAT=docker podman build \
+#   --build-arg BUILD_DATE="$(date -u +%Y-%m-%dT%H:%M:%SZ)" \
+#   --build-arg GIT_REVISION="$(git describe --always --dirty)" \
+#   -t favoritter .
+
+FROM docker.io/library/golang:1.23-bookworm AS builder
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+ARG VERSION=dev
+ARG BUILD_DATE=unknown
+RUN CGO_ENABLED=0 go build \
+    -ldflags="-s -w -X main.version=${VERSION} -X main.buildDate=${BUILD_DATE}" \
+    -o /favoritter ./cmd/favoritter
+
+FROM docker.io/library/debian:bookworm-slim
+ARG BUILD_DATE
+ARG GIT_REVISION
+LABEL org.opencontainers.image.created="${BUILD_DATE}" \
+      org.opencontainers.image.revision="${GIT_REVISION}" \
+      org.opencontainers.image.source="https://kode.naiv.no/olemd/favoritter" \
+      org.opencontainers.image.licenses="AGPL-3.0-or-later" \
+      org.opencontainers.image.title="Favoritter" \
+      org.opencontainers.image.description="Self-hosted favorites web app"
+RUN printf 'build_date=%s\ngit_revision=%s\n' "${BUILD_DATE}" "${GIT_REVISION}" > /etc/build-info
+
+RUN useradd -r -s /usr/sbin/nologin favoritter \
+    && mkdir -p /data/uploads \
+    && chown -R favoritter:favoritter /data
+USER favoritter
+COPY --from=builder /favoritter /usr/local/bin/favoritter
+
+ENV FAVORITTER_DB_PATH=/data/favoritter.db \
+    FAVORITTER_UPLOAD_DIR=/data/uploads \
+    FAVORITTER_LISTEN=:8080
+
+EXPOSE 8080
+VOLUME ["/data"]
+HEALTHCHECK --interval=30s --timeout=3s CMD ["/usr/local/bin/favoritter", "-healthcheck"]
+ENTRYPOINT ["/usr/local/bin/favoritter"]