Refactor: Align system with gateway_users architecture

- Fixed database relationships: LlmRequest now properly uses gateway_user_id instead of user_id
- Updated Models: GatewayUser and LlmRequest relationships corrected
- Removed User->llmRequests relationship (admin users don't have LLM requests)
- Simplified Dashboard: Now shows Gateway User statistics instead of admin users
- Removed obsolete Budgets management pages (budgets handled directly in gateway_users)
- Removed User Budgets admin section (redundant with gateway_users management)
- Fixed view errors: Added null-checks for user_id in keys views
- Updated navigation: Removed Budget and User Budget links
- Updated routes: Cleaned up unused BudgetController and UserManagementController routes
- Simplified StatisticsService: Focus on gateway_users and basic metrics only

laravel-app/app/Models/LlmRequest.php

@@ -8,7 +8,7 @@ use Illuminate\Database\Eloquent\Relations\BelongsTo;
 class LlmRequest extends Model
 {
     protected $fillable = [
-        'user_id',
+        'gateway_user_id',
         'provider',
         'model',
         'request_payload',
@@ -41,9 +41,9 @@ class LlmRequest extends Model
         'http_status' => 'integer',
     ];
 
-    public function user(): BelongsTo
+    public function gatewayUser(): BelongsTo
     {
-        return $this->belongsTo(User::class);
+        return $this->belongsTo(GatewayUser::class, 'gateway_user_id', 'user_id');
     }
 
     public function isSuccess(): bool