feat(auth): 实现 TOTP 双因素认证功能

新增功能：
- 支持 Google Authenticator 等应用进行 TOTP 二次验证
- 用户可在个人设置中启用/禁用 2FA
- 登录时支持 TOTP 验证流程
- 管理后台可全局开关 TOTP 功能

安全增强：
- TOTP 密钥使用 AES-256-GCM 加密存储
- 添加 TOTP_ENCRYPTION_KEY 配置项，必须手动配置才能启用功能
- 防止服务重启导致加密密钥变更使用户无法登录
- 验证失败次数限制，防止暴力破解

配置说明：
- Docker 部署：在 .env 中设置 TOTP_ENCRYPTION_KEY
- 非 Docker 部署：在 config.yaml 中设置 totp.encryption_key
- 生成密钥命令：openssl rand -hex 32

backend/internal/repository/aes_encryptor.go

@@ -0,0 +1,95 @@
+package repository
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"encoding/base64"
+	"encoding/hex"
+	"fmt"
+	"io"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/Wei-Shaw/sub2api/internal/service"
+)
+
+// AESEncryptor implements SecretEncryptor using AES-256-GCM
+type AESEncryptor struct {
+	key []byte
+}
+
+// NewAESEncryptor creates a new AES encryptor
+func NewAESEncryptor(cfg *config.Config) (service.SecretEncryptor, error) {
+	key, err := hex.DecodeString(cfg.Totp.EncryptionKey)
+	if err != nil {
+		return nil, fmt.Errorf("invalid totp encryption key: %w", err)
+	}
+
+	if len(key) != 32 {
+		return nil, fmt.Errorf("totp encryption key must be 32 bytes (64 hex chars), got %d bytes", len(key))
+	}
+
+	return &AESEncryptor{key: key}, nil
+}
+
+// Encrypt encrypts plaintext using AES-256-GCM
+// Output format: base64(nonce + ciphertext + tag)
+func (e *AESEncryptor) Encrypt(plaintext string) (string, error) {
+	block, err := aes.NewCipher(e.key)
+	if err != nil {
+		return "", fmt.Errorf("create cipher: %w", err)
+	}
+
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return "", fmt.Errorf("create gcm: %w", err)
+	}
+
+	// Generate a random nonce
+	nonce := make([]byte, gcm.NonceSize())
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return "", fmt.Errorf("generate nonce: %w", err)
+	}
+
+	// Encrypt the plaintext
+	// Seal appends the ciphertext and tag to the nonce
+	ciphertext := gcm.Seal(nonce, nonce, []byte(plaintext), nil)
+
+	// Encode as base64
+	return base64.StdEncoding.EncodeToString(ciphertext), nil
+}
+
+// Decrypt decrypts ciphertext using AES-256-GCM
+func (e *AESEncryptor) Decrypt(ciphertext string) (string, error) {
+	// Decode from base64
+	data, err := base64.StdEncoding.DecodeString(ciphertext)
+	if err != nil {
+		return "", fmt.Errorf("decode base64: %w", err)
+	}
+
+	block, err := aes.NewCipher(e.key)
+	if err != nil {
+		return "", fmt.Errorf("create cipher: %w", err)
+	}
+
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return "", fmt.Errorf("create gcm: %w", err)
+	}
+
+	nonceSize := gcm.NonceSize()
+	if len(data) < nonceSize {
+		return "", fmt.Errorf("ciphertext too short")
+	}
+
+	// Extract nonce and ciphertext
+	nonce, ciphertextData := data[:nonceSize], data[nonceSize:]
+
+	// Decrypt
+	plaintext, err := gcm.Open(nil, nonce, ciphertextData, nil)
+	if err != nil {
+		return "", fmt.Errorf("decrypt: %w", err)
+	}
+
+	return string(plaintext), nil
+}