feat: make user limit configurable from admin user management (#18)

Store the per-instance user cap in settings (default 20) and expose
GET/PUT /admin/users/limit endpoints. The user management page now lets
admins view and update the limit without touching system settings.

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: truewhile <truewhile@users.noreply.github.com>

internal/service/auth.go

@@ -40,14 +40,8 @@ var (
 	ErrUserExpired        = errors.New("user account has expired")
 )
 
-// MaxUsers 是单实例允许的最大用户数（开源版本固定上限）。
-const MaxUsers = OpenSourceUserLimit
-
-// OpenSourceUserLimit 是开源版本的用户数上限。
-const OpenSourceUserLimit = 20
-
-// UserLimit 是注册/邀请码发放时的固定用户数上限（授权管理已移除，固定为开源上限）。
-const UserLimit = OpenSourceUserLimit
+// MaxUsers is kept for tests that seed up to the default cap.
+const MaxUsers = DefaultMaxUsers
 
 // SeedAdmin makes sure at least one admin user exists. It mirrors the
 // legacy default behaviour: if no admin row is found we create
@@ -103,9 +97,13 @@ func (s *AuthService) Register(ctx context.Context, username, password string) (
 	if err := s.repo.User.ReleaseDeletedUsername(ctx, username); err != nil {
 		return nil, nil, err
 	}
+	limit, err := LoadMaxUsers(ctx, s.repo)
+	if err != nil {
+		return nil, nil, err
+	}
 	if n, err := s.repo.User.Count(ctx); err != nil {
 		return nil, nil, err
-	} else if n >= UserLimit {
+	} else if n >= int64(limit) {
 		return nil, nil, ErrUserLimitReached
 	}
 	hash, err := hashPassword(password)