refactor: 提取 avatar handler 魔法数字为具名常量

- maxAvatarSize = 5 * 1024 * 1024 (5MB)
- magicBytesBufSize = 512
- avatarTokenLen = 8
- dirPerm = 0o755
- filePerm = 0o644

internal/api/handler/avatar_handler.go

@@ -32,6 +32,14 @@ func NewAvatarHandler(userRepo avatarUserRepository) *AvatarHandler {
 	return &AvatarHandler{userRepo: userRepo}
 }
 
+const (
+	maxAvatarSize     = 5 * 1024 * 1024 // 5MB
+	magicBytesBufSize = 512
+	avatarTokenLen    = 8
+	dirPerm           = 0o755
+	filePerm          = 0o644
+)
+
 // generateSecureToken generates a secure random token
 func generateSecureToken(length int) string {
 	bytes := make([]byte, length)
@@ -93,7 +101,7 @@ func (h *AvatarHandler) UploadAvatar(c *gin.Context) {
 	}
 
 	// Validate file size (max 5MB)
-	if file.Size > 5*1024*1024 {
+	if file.Size > maxAvatarSize {
 		c.JSON(http.StatusBadRequest, gin.H{"code": 400, "message": "file size exceeds 5MB limit"})
 		return
 	}
@@ -115,7 +123,7 @@ func (h *AvatarHandler) UploadAvatar(c *gin.Context) {
 	defer src.Close()
 
 	// Validate Magic Bytes to detect actual file type (prevents file extension spoofing)
-	buf := make([]byte, 512)
+	buf := make([]byte, magicBytesBufSize)
 	n, err := src.Read(buf)
 	if err != nil && err != io.EOF {
 		c.JSON(http.StatusBadRequest, gin.H{"code": 400, "message": "failed to read file"})
@@ -140,11 +148,11 @@ func (h *AvatarHandler) UploadAvatar(c *gin.Context) {
 	}
 
 	// Generate unique filename
-	avatarFilename := fmt.Sprintf("avatar_%d_%s%s", userID, generateSecureToken(8), ext)
+	avatarFilename := fmt.Sprintf("avatar_%d_%s%s", userID, generateSecureToken(avatarTokenLen), ext)
 	uploadDir := "./uploads/avatars"
 
 	// Create upload directory if not exists
-	if err := os.MkdirAll(uploadDir, 0o755); err != nil {
+	if err := os.MkdirAll(uploadDir, dirPerm); err != nil {
 		c.JSON(http.StatusInternalServerError, gin.H{"code": 500, "message": "failed to create upload directory"})
 		return
 	}
@@ -156,7 +164,7 @@ func (h *AvatarHandler) UploadAvatar(c *gin.Context) {
 		c.JSON(http.StatusInternalServerError, gin.H{"code": 500, "message": "failed to read uploaded file"})
 		return
 	}
-	if err := os.WriteFile(dstPath, data, 0o644); err != nil {
+	if err := os.WriteFile(dstPath, data, filePerm); err != nil {
 		c.JSON(http.StatusInternalServerError, gin.H{"code": 500, "message": "failed to save avatar file"})
 		return
 	}