fix: resolve 13 release-consistency findings

Documentation:
- Fix CI badge href to /actions (was 404 on Gitea)
- Add UPTOP_METRICS_PUBLIC + UPTOP_MAINT_RETENTION to README env table
- Link maintenance retention to env var name in data retention section
- Note metrics auth requirement in features list
- Fix clustering.md: fail-closed wording, mark AGG_STRATEGY/NODE_REGION optional
- Fix .env.example: wording (no .env loader), add TRUSTED_PROXIES + MAINT_RETENTION
- Add CLI help/usage with subcommand listing, accept serve/help/-h/-version

Docker/deploy:
- Add EXPOSE 8080 to Dockerfile
- Remove dead LIPGLOSS_RENDERER_HAS_DARK_BACKGROUND env
- Exempt /api/health from cluster auth (fixes Docker HEALTHCHECK 401)
- Add sysctls for unprivileged ping to all compose files

Cosmetic:
- Fix bug_report.yaml: SemVer placeholder, remove nonexistent serve subcommand

Dockerfile

@@ -23,14 +23,13 @@ RUN mkdir -p /data/.ssh && chown -R uptop:uptop /data
 COPY --from=builder /app/uptop .
 COPY --chmod=755 docker-entrypoint.sh /usr/local/bin/
 
-ENV LIPGLOSS_RENDERER_HAS_DARK_BACKGROUND=true
 ENV UPTOP_DB_TYPE=sqlite
 ENV UPTOP_DB_DSN=/data/uptop.db
 ENV UPTOP_KEYS=/data/authorized_keys
 ENV UPTOP_SSH_HOST_KEY=/data/.ssh/id_ed25519
 ENV UPTOP_PORT=23234
 
-EXPOSE 23234
+EXPOSE 8080 23234
 HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
   CMD wget -qO- http://localhost:8080/api/health || exit 1
 USER uptop