Add Google OAuth authentication support

- Add Google OAuth token verification and user lookup/creation
- Add GoogleAuthRequest and GoogleAuthResponse DTOs
- Add GoogleLogin handler in auth_handler.go
- Add google_auth.go service for token verification
- Add FindByGoogleID repository method for user lookup
- Add GoogleID field to User model
- Add Google OAuth configuration (client ID, enabled flag)
- Add i18n translations for Google auth error messages
- Add Google verification email template support

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/models/user.go

@@ -247,3 +247,21 @@ type AppleSocialAuth struct {
 func (AppleSocialAuth) TableName() string {
 	return "user_applesocialauth"
 }
+
+// GoogleSocialAuth represents a user's linked Google account for Sign in with Google
+type GoogleSocialAuth struct {
+	ID        uint      `gorm:"primaryKey" json:"id"`
+	UserID    uint      `gorm:"uniqueIndex;not null" json:"user_id"`
+	User      User      `gorm:"foreignKey:UserID" json:"-"`
+	GoogleID  string    `gorm:"column:google_id;size:255;uniqueIndex;not null" json:"google_id"` // Google's unique subject ID
+	Email     string    `gorm:"column:email;size:254" json:"email"`
+	Name      string    `gorm:"column:name;size:255" json:"name"`
+	Picture   string    `gorm:"column:picture;size:512" json:"picture"` // Profile picture URL
+	CreatedAt time.Time `gorm:"column:created_at;autoCreateTime" json:"created_at"`
+	UpdatedAt time.Time `gorm:"column:updated_at;autoUpdateTime" json:"updated_at"`
+}
+
+// TableName returns the table name for GORM
+func (GoogleSocialAuth) TableName() string {
+	return "user_googlesocialauth"
+}