feat: add GORM integration with repository pattern

- Add GORM models in models/ directory with proper column tags
- Create repository interfaces and implementations in core/repository/
- Add database package with MySQL and SQLite support
- Add UUID ID utility for GORM models
- Implement complete repository layer replacing SQL-based data access
- Add database migrations and index creation
- Support both MySQL and SQLite drivers with auto-migration

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

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

models/balance.go

@@ -0,0 +1,11 @@
+package models
+
+// Balance represents an account balance at a point in time
+type Balance struct {
+	ID        uint   `gorm:"primaryKey;autoIncrement"`
+	Date      uint64 `gorm:"column:date;not null"`
+	AccountID []byte `gorm:"column:accountId;type:BINARY(16);not null"`
+	Amount    int64  `gorm:"column:amount;not null"`
+
+	Account Account `gorm:"foreignKey:AccountID"`
+}