refactor(auth): move authentication params to headers and simplify validation

- Move token and timestamp validation to HTTP headers
- Simplify ValidateTimeStamp to return boolean
- Update AddUser to use default "visitor" type
- Remove redundant timestamp and token fields from request structs
- Update API documentation to reflect header-based authentication

database.go

@@ -42,7 +42,7 @@ func isValidInput(input string) bool {
 	return true
 }
 
-func AddUser(username, passwd, userType string) (int, error) {
+func AddUser(username, passwd string) (int, error) { // New user registration with default type "visitor"
 	if !isValidInput(username) || !isValidInput(passwd) {
 		return 0, errors.New("invalid input: contains illegal characters")
 	}
@@ -57,7 +57,7 @@ func AddUser(username, passwd, userType string) (int, error) {
 	}
 
 	result, err := db.Exec("INSERT INTO userLogin (username, passwd, type) VALUES (?, ?, ?)",
-		username, hashedPasswd, userType)
+		username, hashedPasswd, "visitor")
 	if err != nil {
 		if strings.Contains(err.Error(), "UNIQUE constraint failed") {
 			return 0, errors.New("username already exists")