Store signal quality score in database for future analysis

- Add signalQualityScore field to Trade model (0-100)
- Calculate quality score in execute endpoint using same logic as check-risk
- Save score with every trade for correlation analysis
- Create database migration for new field
- Enables future analysis: score vs win rate, P&L, etc.

This allows data-driven decisions on dynamic position sizing

prisma/migrations/20251031100807_add_signal_quality_score/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "Trade" ADD COLUMN     "signalQualityScore" INTEGER;