feat: apply category defaults first then AI adjusts + remove chips

Backend: merge strategy — category defaults are applied as baseline
(ai_generated=0), AI impacts override/extend them (ai_generated=1).
All N category defaults appear in the impacts list; AI covers them
explicitly with event-specific score/direction adjustments.
Prompt: instruct AI to cover ALL default instruments.

Frontend: remove default impact chips under category selector —
they now appear directly in the instrument impacts list.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/MarketEvents.tsx

@@ -502,20 +502,6 @@ function EventDetail({
               {matchedCat?.description && (
                 <div className="text-xs text-slate-600 mt-1.5 italic">{matchedCat.description}</div>
               )}
-              {matchedCat && nDefaults > 0 && (
-                <div className="flex flex-wrap gap-1 mt-2">
-                  {matchedCat.default_impacts.slice(0, 8).map((di: DefaultImpact) => (
-                    <span key={di.instrument_id}
-                      className={clsx('text-xs px-1.5 py-0.5 rounded border font-mono',
-                        di.typical_direction === 'bullish'  ? 'text-emerald-400 border-emerald-800/40 bg-emerald-900/20' :
-                        di.typical_direction === 'bearish'  ? 'text-red-400 border-red-800/40 bg-red-900/20' :
-                        'text-slate-400 border-slate-700 bg-slate-800/40')}>
-                      {di.typical_direction === 'bullish' ? '↑' : di.typical_direction === 'bearish' ? '↓' : '–'} {di.instrument_id}
-                      <span className="text-slate-600 ml-1">{(di.sensitivity * 100).toFixed(0)}%</span>
-                    </span>
-                  ))}
-                </div>
-              )}
             </div>
           )
         })()}