Fix: SQLX type conversion in ml_performance_metrics.rs

Issue: Type mismatch between Decimal and BigDecimal in PnL recording
Root Cause: SQLX configured with rust_decimal, not bigdecimal
Fix: Remove ::numeric cast, use Decimal directly (SQLX native support)

Changes:
- Remove bigdecimal imports and conversion logic
- SQLX query now uses Decimal directly (line 114)
- Regenerated SQLX prepared query cache
- trading_service library compiles successfully

Testing:
- cargo check -p trading_service ✅ (library only)
- SQLX offline mode ✅ (queries cached)
- 35 warnings (non-blocking, clippy suggestions)

Status: Compilation blocker resolved → Wave 17 ready

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Cargo.lock

@@ -1691,6 +1691,7 @@ dependencies = [
  "num-bigint 0.4.6",
  "num-integer",
  "num-traits",
+ "serde",
 ]
 
 [[package]]
@@ -10350,6 +10351,7 @@ dependencies = [
  "axum 0.7.9",
  "base32",
  "base64 0.22.1",
+ "bigdecimal",
  "bytes",
  "chrono",
  "clap 4.5.48",