feat: migrate store layer to GORM with PostgreSQL support

- Migrate all store packages from raw database/sql to GORM ORM
- Add PostgreSQL support alongside SQLite
- Move EncryptedString type to crypto package for cleaner architecture
- Add automatic encryption/decryption for sensitive fields (API keys, secrets)
- Fix PostgreSQL AutoMigrate conflicts by skipping existing tables
- Fix duplicate /klines route registration
- Update tests to use GORM database connections
- Add database configuration support in config package

backtest/retention.go

@@ -76,8 +76,8 @@ func enforceRetentionDB(maxRuns int) {
 	query := `
 		SELECT run_id FROM backtest_runs
 		WHERE state IN (?, ?, ?, ?)
-		ORDER BY datetime(updated_at) DESC
-		LIMIT -1 OFFSET ?
+		ORDER BY updated_at DESC
+		OFFSET ?
 	`
 	rows, err := persistenceDB.Query(query,
 		finalStates[0], finalStates[1], finalStates[2], finalStates[3], maxRuns)