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/user.go

@@ -0,0 +1,21 @@
+package models
+
+// User represents a user account
+type User struct {
+	ID              []byte         `gorm:"type:BINARY(16);primaryKey"`
+	Inserted        uint64         `gorm:"column:inserted;not null"`
+	Updated         uint64         `gorm:"column:updated;not null"`
+	FirstName       string         `gorm:"column:firstName;size:50;not null"`
+	LastName        string         `gorm:"column:lastName;size:50;not null"`
+	Email           string         `gorm:"column:email;size:100;not null;unique"`
+	PasswordHash    string         `gorm:"column:passwordHash;size:100;not null"`
+	AgreeToTerms    bool           `gorm:"column:agreeToTerms;not null"`
+	PasswordReset   string         `gorm:"column:passwordReset;size:32;not null"`
+	EmailVerified   bool           `gorm:"column:emailVerified;not null"`
+	EmailVerifyCode string         `gorm:"column:emailVerifyCode;size:32;not null"`
+	SignupSource    string         `gorm:"column:signupSource;size:100;not null"`
+
+	UserOrgs []UserOrg `gorm:"foreignKey:UserID"`
+	Sessions []Session `gorm:"foreignKey:UserID"`
+	APIKeys  []APIKey  `gorm:"foreignKey:UserID"`
+}