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/config/config.go

@@ -13,15 +13,16 @@ import (
 
 // Config holds all configuration for the application
 type Config struct {
-	Server    ServerConfig
-	Database  DatabaseConfig
-	Redis     RedisConfig
-	Email     EmailConfig
-	Push      PushConfig
-	Worker    WorkerConfig
-	Security  SecurityConfig
-	Storage   StorageConfig
-	AppleAuth AppleAuthConfig
+	Server     ServerConfig
+	Database   DatabaseConfig
+	Redis      RedisConfig
+	Email      EmailConfig
+	Push       PushConfig
+	Worker     WorkerConfig
+	Security   SecurityConfig
+	Storage    StorageConfig
+	AppleAuth  AppleAuthConfig
+	GoogleAuth GoogleAuthConfig
 }
 
 type ServerConfig struct {
@@ -78,6 +79,12 @@ type AppleAuthConfig struct {
 	TeamID   string // Apple Developer Team ID
 }
 
+type GoogleAuthConfig struct {
+	ClientID        string // Web client ID for token verification
+	AndroidClientID string // Android client ID (optional, for audience verification)
+	IOSClientID     string // iOS client ID (optional, for audience verification)
+}
+
 type WorkerConfig struct {
 	// Scheduled job times (UTC)
 	TaskReminderHour    int
@@ -194,6 +201,11 @@ func Load() (*Config, error) {
 			ClientID: viper.GetString("APPLE_CLIENT_ID"),
 			TeamID:   viper.GetString("APPLE_TEAM_ID"),
 		},
+		GoogleAuth: GoogleAuthConfig{
+			ClientID:        viper.GetString("GOOGLE_CLIENT_ID"),
+			AndroidClientID: viper.GetString("GOOGLE_ANDROID_CLIENT_ID"),
+			IOSClientID:     viper.GetString("GOOGLE_IOS_CLIENT_ID"),
+		},
 	}
 
 	// Validate required fields