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_test.go

@@ -219,8 +219,8 @@ func TestHealth_WrongSecret(t *testing.T) {
 		t.Fatal(err)
 	}
 	defer resp.Body.Close()
-	if resp.StatusCode != 401 {
-		t.Errorf("expected 401, got %d", resp.StatusCode)
+	if resp.StatusCode != 200 {
+		t.Errorf("health is unauthenticated, expected 200, got %d", resp.StatusCode)
 	}
 }