fix: backtest module PostgreSQL compatibility and bug fixes

- Fix PostgreSQL placeholder conversion (? to $1, $2...) in all SQL queries
- Fix int4 overflow for timestamp columns (ALTER to BIGINT)
- Fix notional calculation bug in position Close() using proportional entry
- Fix potential panic in DecisionTimestamp with bounds check
- Fix nil pointer dereference in sliceUpTo with defensive checks
- Fix race condition in releaseLock using sync.Once
- Fix UnrealizedPnLPct always 0 in convertPositions
- Improve Sharpe ratio calculation with proper negative return handling

backtest/persistence_db.go

@@ -2,15 +2,39 @@ package backtest
 
 import (
 	"database/sql"
+	"fmt"
+	"strings"
 )
 
 var persistenceDB *sql.DB
+var dbIsPostgres bool
 
 // UseDatabase enables database-backed persistence for all backtest storage operations.
+// If isPostgres is true, queries will use $1, $2... placeholders instead of ?
 func UseDatabase(db *sql.DB) {
 	persistenceDB = db
 }
 
+// UseDatabaseWithType enables database-backed persistence with explicit type.
+func UseDatabaseWithType(db *sql.DB, isPostgres bool) {
+	persistenceDB = db
+	dbIsPostgres = isPostgres
+}
+
 func usingDB() bool {
 	return persistenceDB != nil
 }
+
+// convertQuery converts ? placeholders to $1, $2, etc. for PostgreSQL
+func convertQuery(query string) string {
+	if !dbIsPostgres {
+		return query
+	}
+	result := query
+	index := 1
+	for strings.Contains(result, "?") {
+		result = strings.Replace(result, "?", fmt.Sprintf("$%d", index), 1)
+		index++
+	}
+	return result
+}