Fix API controllers to use correct database column names

- Fix model_pricing table references (model_id -> model, display_name -> model)
- Fix price columns (output_price_per_1k -> output_price_per_million)
- Add price conversion (per_million / 1000 = per_1k) in all API responses
- Add whereNotNull('model') filters to exclude invalid entries
- Add getModelDisplayName() helper method to all controllers
- Fix AccountController to use gateway_users budget fields directly
- Remove Budget model dependencies from AccountController
- Add custom Scramble server URL configuration for API docs
- Create ScrambleServiceProvider to set correct /api prefix
- Add migration to rename user_id to gateway_user_id in llm_requests
- Add custom ApiGuard for gateway_users authentication
- Update all API controllers: AccountController, ModelController, PricingController, ProviderController

All API endpoints now working correctly:
- GET /api/account
- GET /api/models
- GET /api/pricing
- GET /api/providers/{provider}

laravel-app/app/Providers/ScrambleServiceProvider.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace App\Providers;
+
+use Dedoc\Scramble\Scramble;
+use Illuminate\Support\ServiceProvider;
+
+class ScrambleServiceProvider extends ServiceProvider
+{
+    public function register(): void
+    {
+        //
+    }
+
+    public function boot(): void
+    {
+        Scramble::extendOpenApi(function ($openApi) {
+            $openApi->servers = [
+                \Dedoc\Scramble\Support\Generator\Server::make(url('/api')),
+            ];
+        });
+    }
+}