Enhance API endpoints with response type annotations

- Updated various API endpoints to include response type annotations using .Produces<T>() for better documentation and type safety.
- Enhanced activation, admin, user, config, and other endpoints to specify response types, improving clarity for frontend integration.
- Added new DTOs for structured responses in authentication and Telegram-related endpoints.
- Improved overall API schema generation to reflect these changes, ensuring consistency between backend and frontend types.

backend/src/PnvPanel.Api/Endpoints/SubscriptionEndpoints.cs

@@ -12,7 +12,9 @@ public static class SubscriptionEndpoints
         // Вне /api по дизайну (api-design.md) — публичный эндпоинт для VPN-клиентов.
         app.MapGet("/sub/{token}", GetSubscription)
             .WithTags("Subscription")
-            .RequireRateLimiting(RateLimiting.AuthPolicy);
+            .RequireRateLimiting(RateLimiting.AuthPolicy)
+            .Produces<string>(StatusCodes.Status200OK, "text/plain")
+            .Produces(StatusCodes.Status404NotFound);
 
         return app;
     }