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/dto/requests/auth.go

@@ -58,3 +58,8 @@ type AppleSignInRequest struct {
 	FirstName *string `json:"first_name"`
 	LastName  *string `json:"last_name"`
 }
+
+// GoogleSignInRequest represents the Google Sign In request body
+type GoogleSignInRequest struct {
+	IDToken string `json:"id_token" binding:"required"` // Google ID token from Credential Manager
+}

internal/dto/responses/auth.go

@@ -171,3 +171,19 @@ func NewAppleSignInResponse(token string, user *models.User, isNewUser bool) App
 		IsNewUser: isNewUser,
 	}
 }
+
+// GoogleSignInResponse represents the Google Sign In response
+type GoogleSignInResponse struct {
+	Token     string       `json:"token"`
+	User      UserResponse `json:"user"`
+	IsNewUser bool         `json:"is_new_user"`
+}
+
+// NewGoogleSignInResponse creates a GoogleSignInResponse
+func NewGoogleSignInResponse(token string, user *models.User, isNewUser bool) GoogleSignInResponse {
+	return GoogleSignInResponse{
+		Token:     token,
+		User:      NewUserResponse(user),
+		IsNewUser: isNewUser,
+	}
+}