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/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))
+}