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

internal/server/server.go

@@ -159,10 +159,6 @@ func (s *Server) handleHealth(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
 		return
 	}
-	if s.cfg.ClusterKey != "" && !checkSecret(r.Header.Get("X-Uptop-Secret"), s.cfg.ClusterKey) {
-		http.Error(w, "Unauthorized", http.StatusUnauthorized)
-		return
-	}
 	w.WriteHeader(http.StatusOK)
 	_, _ = w.Write([]byte("OK"))
 }