Cleanup retired S/R research scaffolding

- Remove unused _gate_eligible_levels filtering logic and its tests (research-only)
- Add prominent RESEARCH/DIAGNOSTIC markers and docs to clear-air/ATR fallback helpers
- Document production vs research BACKTEST_* environment variables in backtest_service
- Minor cleanups: update legacy report text, improve outdated function docstring

frontend/src/components/signals/BacktestPanel.tsx

@@ -263,7 +263,7 @@ export function BacktestPanel() {
               )}
               {' '}· target model:{' '}
               <span className={report.params.is_production_target_model === false ? 'text-amber-300' : 'text-blue-300'}>
-                {report.params.target_model_label ?? 'Legacy report (model not recorded)'}
+                {report.params.target_model_label ?? 'Unknown (legacy report)'}
               </span>
             </p>