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/api_key.go

@@ -0,0 +1,13 @@
+package models
+
+// APIKey represents API keys for users
+type APIKey struct {
+	ID       []byte `gorm:"type:BINARY(16);primaryKey"`
+	Inserted uint64 `gorm:"column:inserted;not null"`
+	Updated  uint64 `gorm:"column:updated;not null"`
+	UserID   []byte `gorm:"column:userId;type:BINARY(16);not null"`
+	Label    string `gorm:"column:label;size:300;not null"`
+	Deleted  uint64 `gorm:"column:deleted"`
+
+	User User `gorm:"foreignKey:UserID"`
+}