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/base.go

@@ -0,0 +1,45 @@
+package models
+
+import (
+	"github.com/google/uuid"
+	"github.com/openaccounting/oa-server/core/util/id"
+	"gorm.io/gorm"
+)
+
+type Base struct {
+	ID []byte `gorm:"type:BINARY(16);primaryKey"`
+}
+
+// GetUUID converts binary ID to UUID
+func (b *Base) GetUUID() (uuid.UUID, error) {
+	return id.ToUUID(b.ID)
+}
+
+// GetIDString returns string representation of the ID
+func (b *Base) GetIDString() string {
+	return id.String(b.ID)
+}
+
+// SetIDFromString parses string UUID into binary ID
+func (b *Base) SetIDFromString(s string) error {
+	binID, err := id.FromString(s)
+	if err != nil {
+		return err
+	}
+	b.ID = binID
+	return nil
+}
+
+// ValidateID checks if the ID is a valid UUID
+func (b *Base) ValidateID() error {
+	_, err := uuid.FromBytes(b.ID)
+	return err
+}
+
+// BeforeCreate GORM hook to set ID if empty
+func (b *Base) BeforeCreate(tx *gorm.DB) error {
+	if len(b.ID) == 0 {
+		b.ID = id.New()
+	}
+	return nil
+}