fix(web): refuse oversized login usernames before recording them

A failed login keeps its username in the in-memory log buffer, the
limiter table and the notifier queue, and the route accepted any body
the global cap allowed. The login route now takes at most 16 KiB and
answers a username over 256 bytes with the generic failure before any
of those see it; the user setters refuse such a name too, so no stored
username can be one the login route turns away.

internal/web/service/panel/user.go

@@ -2,6 +2,7 @@ package panel
 
 import (
 	"errors"
+	"fmt"
 	"time"
 
 	"gorm.io/gorm"
@@ -128,6 +129,9 @@ func (s *UserService) BumpLoginEpoch() error {
 }
 
 func (s *UserService) UpdateUser(id int, username string, password string) error {
+	if len(username) > MaxUsernameLength {
+		return fmt.Errorf("username can not be longer than %d bytes", MaxUsernameLength)
+	}
 	db := database.GetDB()
 	hashedPassword, err := crypto.HashPasswordAsBcrypt(password)
 	if err != nil {
@@ -157,6 +161,8 @@ func (s *UserService) UpdateUser(id int, username string, password string) error
 func (s *UserService) UpdateFirstUser(username string, password string) error {
 	if username == "" {
 		return errors.New("username can not be empty")
+	} else if len(username) > MaxUsernameLength {
+		return fmt.Errorf("username can not be longer than %d bytes", MaxUsernameLength)
 	} else if password == "" {
 		return errors.New("password can not be empty")
 	}
@@ -181,3 +187,7 @@ func (s *UserService) UpdateFirstUser(username string, password string) error {
 	user.LoginEpoch++
 	return db.Save(user).Error
 }
+
+// MaxUsernameLength bounds a panel username: the login route refuses longer names before
+// logging or recording them, so no stored name may exceed it.
+const MaxUsernameLength = 256