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)

internal/service/user_limits.go

@@ -0,0 +1,71 @@
+package service
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"strconv"
+	"strings"
+
+	"github.com/truewhile/MeBox/internal/repository"
+)
+
+const (
+	// SettingMaxUsers stores the per-instance user cap in the settings table.
+	SettingMaxUsers = "user.max_users"
+
+	// DefaultMaxUsers is used when the setting is missing or invalid.
+	DefaultMaxUsers = 20
+
+	// MaxUsersHardCap prevents absurd values from admin input.
+	MaxUsersHardCap = 10000
+)
+
+var (
+	ErrInvalidMaxUsers = errors.New("invalid max users")
+)
+
+// LoadMaxUsers reads the configured user cap, falling back to DefaultMaxUsers.
+func LoadMaxUsers(ctx context.Context, repo *repository.Container) (int, error) {
+	if repo == nil || repo.Setting == nil {
+		return DefaultMaxUsers, nil
+	}
+	raw, err := repo.Setting.Get(ctx, SettingMaxUsers)
+	if err != nil {
+		return DefaultMaxUsers, err
+	}
+	return parseMaxUsersSetting(raw), nil
+}
+
+func parseMaxUsersSetting(raw string) int {
+	n, err := strconv.Atoi(strings.TrimSpace(raw))
+	if err != nil || n < 1 {
+		return DefaultMaxUsers
+	}
+	if n > MaxUsersHardCap {
+		return MaxUsersHardCap
+	}
+	return n
+}
+
+// ValidateMaxUsers checks an admin-provided cap before persisting it.
+func ValidateMaxUsers(n int) error {
+	if n < 1 {
+		return fmt.Errorf("%w: must be at least 1", ErrInvalidMaxUsers)
+	}
+	if n > MaxUsersHardCap {
+		return fmt.Errorf("%w: must be at most %d", ErrInvalidMaxUsers, MaxUsersHardCap)
+	}
+	return nil
+}
+
+// SaveMaxUsers persists the user cap in settings.
+func SaveMaxUsers(ctx context.Context, repo *repository.Container, n int) error {
+	if err := ValidateMaxUsers(n); err != nil {
+		return err
+	}
+	if repo == nil || repo.Setting == nil {
+		return errors.New("settings repository unavailable")
+	}
+	return repo.Setting.Set(ctx, SettingMaxUsers, strconv.Itoa(n))
+}

internal/service/user_limits_test.go

@@ -0,0 +1,67 @@
+package service
+
+import (
+	"context"
+	"fmt"
+	"testing"
+
+	"github.com/truewhile/MeBox/internal/model"
+)
+
+func TestLoadMaxUsersDefaultsToTwenty(t *testing.T) {
+	ctx := context.Background()
+	repos, _, _, _ := newAuthTestServices(t)
+	got, err := LoadMaxUsers(ctx, repos)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != DefaultMaxUsers {
+		t.Fatalf("max users = %d, want %d", got, DefaultMaxUsers)
+	}
+}
+
+func TestSaveAndLoadMaxUsers(t *testing.T) {
+	ctx := context.Background()
+	repos, _, _, _ := newAuthTestServices(t)
+	if err := SaveMaxUsers(ctx, repos, 50); err != nil {
+		t.Fatal(err)
+	}
+	got, err := LoadMaxUsers(ctx, repos)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != 50 {
+		t.Fatalf("max users = %d, want 50", got)
+	}
+}
+
+func TestValidateMaxUsersRejectsInvalid(t *testing.T) {
+	if err := ValidateMaxUsers(0); err == nil {
+		t.Fatal("expected error for 0")
+	}
+	if err := ValidateMaxUsers(MaxUsersHardCap + 1); err == nil {
+		t.Fatal("expected error above hard cap")
+	}
+}
+
+func TestRegisterRespectsConfiguredUserLimit(t *testing.T) {
+	ctx := context.Background()
+	repos, auth, _, _ := newAuthTestServices(t)
+	if err := SaveMaxUsers(ctx, repos, 3); err != nil {
+		t.Fatal(err)
+	}
+	for i := 0; i < 3; i++ {
+		if err := repos.User.Create(ctx, &model.User{
+			Username:     fmt.Sprintf("user-%02d", i),
+			PasswordHash: "hash",
+			Role:         "user",
+			Tier:         "free",
+		}); err != nil {
+			t.Fatal(err)
+		}
+	}
+	_, _, err := auth.Register(ctx, "overflow", "password")
+	if err == nil {
+		t.Fatal("expected user limit error")
+	}
+}