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/account.go

@@ -0,0 +1,18 @@
+package models
+
+// Account represents a financial account
+type Account struct {
+	ID           []byte `gorm:"type:BINARY(16);primaryKey"`
+	OrgID        []byte `gorm:"column:orgId;type:BINARY(16);not null"`
+	Inserted     uint64 `gorm:"column:inserted;not null"`
+	Updated      uint64 `gorm:"column:updated;not null"`
+	Name         string `gorm:"column:name;size:100;not null"`
+	Parent       []byte `gorm:"column:parent;type:BINARY(16);not null"`
+	Currency     string `gorm:"column:currency;size:10;not null"`
+	Precision    int    `gorm:"column:precision;not null"`
+	DebitBalance bool   `gorm:"column:debitBalance;not null"`
+
+	Org      Org       `gorm:"foreignKey:OrgID"`
+	Splits   []Split   `gorm:"foreignKey:AccountID"`
+	Balances []Balance `gorm:"foreignKey:AccountID"`
+}