feat(auth): 添加 Linux DO Connect OAuth 登录支持

- 新增 Linux DO OAuth 配置项和环境变量支持
- 实现 OAuth 授权流程和回调处理
- 前端添加 Linux DO 登录按钮和回调页面
- 支持通过 Linux DO 账号注册/登录
- 添加相关国际化文本

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

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

backend/internal/service/auth_service.go

@@ -2,9 +2,13 @@ package service
 
 import (
 	"context"
+	"crypto/rand"
+	"encoding/hex"
 	"errors"
 	"fmt"
 	"log"
+	"net/mail"
+	"strings"
 	"time"
 
 	"github.com/Wei-Shaw/sub2api/internal/config"
@@ -18,6 +22,7 @@ var (
 	ErrInvalidCredentials  = infraerrors.Unauthorized("INVALID_CREDENTIALS", "invalid email or password")
 	ErrUserNotActive       = infraerrors.Forbidden("USER_NOT_ACTIVE", "user is not active")
 	ErrEmailExists         = infraerrors.Conflict("EMAIL_EXISTS", "email already exists")
+	ErrEmailReserved       = infraerrors.BadRequest("EMAIL_RESERVED", "email is reserved")
 	ErrInvalidToken        = infraerrors.Unauthorized("INVALID_TOKEN", "invalid token")
 	ErrTokenExpired        = infraerrors.Unauthorized("TOKEN_EXPIRED", "token has expired")
 	ErrTokenTooLarge       = infraerrors.BadRequest("TOKEN_TOO_LARGE", "token too large")
@@ -27,6 +32,8 @@ var (
 	ErrServiceUnavailable  = infraerrors.ServiceUnavailable("SERVICE_UNAVAILABLE", "service temporarily unavailable")
 )
 
+const linuxDoSyntheticEmailDomain = "@linuxdo-connect.invalid"
+
 // maxTokenLength 限制 token 大小，避免超长 header 触发解析时的异常内存分配。
 const maxTokenLength = 8192
 
@@ -80,6 +87,11 @@ func (s *AuthService) RegisterWithVerification(ctx context.Context, email, passw
 		return "", nil, ErrRegDisabled
 	}
 
+	// Prevent users from registering emails reserved for synthetic OAuth accounts.
+	if isReservedEmail(email) {
+		return "", nil, ErrEmailReserved
+	}
+
 	// 检查是否需要邮件验证
 	if s.settingService != nil && s.settingService.IsEmailVerifyEnabled(ctx) {
 		// 如果邮件验证已开启但邮件服务未配置，拒绝注册
@@ -161,6 +173,10 @@ func (s *AuthService) SendVerifyCode(ctx context.Context, email string) error {
 		return ErrRegDisabled
 	}
 
+	if isReservedEmail(email) {
+		return ErrEmailReserved
+	}
+
 	// 检查邮箱是否已存在
 	existsEmail, err := s.userRepo.ExistsByEmail(ctx, email)
 	if err != nil {
@@ -195,6 +211,10 @@ func (s *AuthService) SendVerifyCodeAsync(ctx context.Context, email string) (*S
 		return nil, ErrRegDisabled
 	}
 
+	if isReservedEmail(email) {
+		return nil, ErrEmailReserved
+	}
+
 	// 检查邮箱是否已存在
 	existsEmail, err := s.userRepo.ExistsByEmail(ctx, email)
 	if err != nil {
@@ -319,6 +339,101 @@ func (s *AuthService) Login(ctx context.Context, email, password string) (string
 	return token, user, nil
 }
 
+// LoginOrRegisterOAuth logs a user in by email (trusted from an OAuth provider) or creates a new user.
+//
+// This is used by end-user OAuth/SSO login flows (e.g. LinuxDo Connect), and intentionally does
+// NOT require the local password. A random password hash is generated for new users to satisfy
+// the existing database constraint.
+func (s *AuthService) LoginOrRegisterOAuth(ctx context.Context, email, username string) (string, *User, error) {
+	email = strings.TrimSpace(email)
+	if email == "" || len(email) > 255 {
+		return "", nil, infraerrors.BadRequest("INVALID_EMAIL", "invalid email")
+	}
+	if _, err := mail.ParseAddress(email); err != nil {
+		return "", nil, infraerrors.BadRequest("INVALID_EMAIL", "invalid email")
+	}
+
+	username = strings.TrimSpace(username)
+	if len([]rune(username)) > 100 {
+		username = string([]rune(username)[:100])
+	}
+
+	user, err := s.userRepo.GetByEmail(ctx, email)
+	if err != nil {
+		if errors.Is(err, ErrUserNotFound) {
+			// Treat OAuth-first login as registration.
+			if s.settingService != nil && !s.settingService.IsRegistrationEnabled(ctx) {
+				return "", nil, ErrRegDisabled
+			}
+
+			randomPassword, err := randomHexString(32)
+			if err != nil {
+				log.Printf("[Auth] Failed to generate random password for oauth signup: %v", err)
+				return "", nil, ErrServiceUnavailable
+			}
+			hashedPassword, err := s.HashPassword(randomPassword)
+			if err != nil {
+				return "", nil, fmt.Errorf("hash password: %w", err)
+			}
+
+			// Defaults for new users.
+			defaultBalance := s.cfg.Default.UserBalance
+			defaultConcurrency := s.cfg.Default.UserConcurrency
+			if s.settingService != nil {
+				defaultBalance = s.settingService.GetDefaultBalance(ctx)
+				defaultConcurrency = s.settingService.GetDefaultConcurrency(ctx)
+			}
+
+			newUser := &User{
+				Email:        email,
+				Username:     username,
+				PasswordHash: hashedPassword,
+				Role:         RoleUser,
+				Balance:      defaultBalance,
+				Concurrency:  defaultConcurrency,
+				Status:       StatusActive,
+			}
+
+			if err := s.userRepo.Create(ctx, newUser); err != nil {
+				if errors.Is(err, ErrEmailExists) {
+					// Race: user created between GetByEmail and Create.
+					user, err = s.userRepo.GetByEmail(ctx, email)
+					if err != nil {
+						log.Printf("[Auth] Database error getting user after conflict: %v", err)
+						return "", nil, ErrServiceUnavailable
+					}
+				} else {
+					log.Printf("[Auth] Database error creating oauth user: %v", err)
+					return "", nil, ErrServiceUnavailable
+				}
+			} else {
+				user = newUser
+			}
+		} else {
+			log.Printf("[Auth] Database error during oauth login: %v", err)
+			return "", nil, ErrServiceUnavailable
+		}
+	}
+
+	if !user.IsActive() {
+		return "", nil, ErrUserNotActive
+	}
+
+	// Best-effort: fill username when empty.
+	if user.Username == "" && username != "" {
+		user.Username = username
+		if err := s.userRepo.Update(ctx, user); err != nil {
+			log.Printf("[Auth] Failed to update username after oauth login: %v", err)
+		}
+	}
+
+	token, err := s.GenerateToken(user)
+	if err != nil {
+		return "", nil, fmt.Errorf("generate token: %w", err)
+	}
+	return token, user, nil
+}
+
 // ValidateToken 验证JWT token并返回用户声明
 func (s *AuthService) ValidateToken(tokenString string) (*JWTClaims, error) {
 	// 先做长度校验，尽早拒绝异常超长 token，降低 DoS 风险。
@@ -361,6 +476,22 @@ func (s *AuthService) ValidateToken(tokenString string) (*JWTClaims, error) {
 	return nil, ErrInvalidToken
 }
 
+func randomHexString(byteLength int) (string, error) {
+	if byteLength <= 0 {
+		byteLength = 16
+	}
+	buf := make([]byte, byteLength)
+	if _, err := rand.Read(buf); err != nil {
+		return "", err
+	}
+	return hex.EncodeToString(buf), nil
+}
+
+func isReservedEmail(email string) bool {
+	normalized := strings.ToLower(strings.TrimSpace(email))
+	return strings.HasSuffix(normalized, linuxDoSyntheticEmailDomain)
+}
+
 // GenerateToken 生成JWT token
 func (s *AuthService) GenerateToken(user *User) (string, error) {
 	now := time.Now()