Add settings endpoints and registration status check: implement new API endpoints for settings management, including registration status retrieval, and update the registration command handler to enforce registration rules based on site settings.

backend/src/TeleWave.Application/Settings/AppSettingsReader.cs

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+using Microsoft.EntityFrameworkCore;
+using TeleWave.Application.Common.Interfaces;
+
+namespace TeleWave.Application.Settings;
+
+/// <summary>Чтение типизированных значений из key-value настроек с дефолтом при отсутствии ключа.</summary>
+public static class AppSettingsReader
+{
+    public static async Task<bool> GetBoolSettingAsync(
+        this IAppDbContext db,
+        string key,
+        bool defaultValue,
+        CancellationToken cancellationToken
+    )
+    {
+        var value = await db.AppSettings.AsNoTracking()
+            .Where(s => s.Key == key)
+            .Select(s => s.Value)
+            .FirstOrDefaultAsync(cancellationToken);
+
+        return value is not null && bool.TryParse(value, out var parsed) ? parsed : defaultValue;
+    }
+}

backend/src/TeleWave.Application/Settings/GetSiteSettings/GetSiteSettingsQuery.cs

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+using LiteCqrs;
+
+namespace TeleWave.Application.Settings.GetSiteSettings;
+
+public sealed record GetSiteSettingsQuery : IQuery<SiteSettingsDto>;

backend/src/TeleWave.Application/Settings/GetSiteSettings/GetSiteSettingsQueryHandler.cs

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+using LiteCqrs;
+using TeleWave.Application.Common.Interfaces;
+
+namespace TeleWave.Application.Settings.GetSiteSettings;
+
+public sealed class GetSiteSettingsQueryHandler(ISiteSettings siteSettings)
+    : IQueryHandler<GetSiteSettingsQuery, SiteSettingsDto>
+{
+    public async Task<SiteSettingsDto> Handle(
+        GetSiteSettingsQuery query,
+        CancellationToken cancellationToken
+    )
+    {
+        var registrationEnabled = await siteSettings.IsRegistrationEnabledAsync(cancellationToken);
+        return new SiteSettingsDto(registrationEnabled);
+    }
+}

backend/src/TeleWave.Application/Settings/SettingKeys.cs

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+namespace TeleWave.Application.Settings;
+
+/// <summary>Стабильные ключи глобальных настроек сайта в key-value хранилище (AppSetting).</summary>
+public static class SettingKeys
+{
+    /// <summary>Разрешена ли открытая регистрация пользователей (по умолчанию — нет).</summary>
+    public const string RegistrationEnabled = "registration.enabled";
+}

backend/src/TeleWave.Application/Settings/SiteSettingsDto.cs

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+namespace TeleWave.Application.Settings;
+
+/// <summary>Глобальные настройки сайта, управляемые администратором.</summary>
+public sealed record SiteSettingsDto(bool RegistrationEnabled);

backend/src/TeleWave.Application/Settings/UpdateSiteSettings/UpdateSiteSettingsCommand.cs

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+using LiteCqrs;
+using TeleWave.Application.Common.Models;
+
+namespace TeleWave.Application.Settings.UpdateSiteSettings;
+
+public sealed record UpdateSiteSettingsCommand(bool RegistrationEnabled) : ICommand<Result>;

backend/src/TeleWave.Application/Settings/UpdateSiteSettings/UpdateSiteSettingsCommandHandler.cs

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+using LiteCqrs;
+using TeleWave.Application.Common.Interfaces;
+using TeleWave.Application.Common.Models;
+
+namespace TeleWave.Application.Settings.UpdateSiteSettings;
+
+public sealed class UpdateSiteSettingsCommandHandler(ISiteSettings siteSettings)
+    : ICommandHandler<UpdateSiteSettingsCommand, Result>
+{
+    public async Task<Result> Handle(
+        UpdateSiteSettingsCommand command,
+        CancellationToken cancellationToken
+    )
+    {
+        // Сохранение выполняет UnitOfWorkBehavior команды.
+        await siteSettings.SetRegistrationEnabledAsync(command.RegistrationEnabled, cancellationToken);
+        return Result.Success();
+    }
+}