Fix: Remove non-existent BillingConfiguration from billing admin

BillingConfiguration model doesn't exist in the codebase, causing ImportError
and preventing backend from starting. Removed the import and admin class.

This was causing the backend to be stuck in restarting state.

backend/igny8_core/modules/billing/admin.py

@@ -9,7 +9,6 @@ from simple_history.admin import SimpleHistoryAdmin
 from igny8_core.admin.base import AccountAdminMixin, Igny8ModelAdmin
 from igny8_core.business.billing.models import (
     CreditCostConfig,
-    BillingConfiguration,
     Invoice,
     Payment,
     CreditPackage,
@@ -702,51 +701,6 @@ class PlanLimitUsageAdmin(ExportMixin, AccountAdminMixin, Igny8ModelAdmin):
     bulk_delete_old_records.short_description = 'Delete old records (>1 year)'
 
 
-@admin.register(BillingConfiguration)
-class BillingConfigurationAdmin(Igny8ModelAdmin):
-    """Admin for global billing configuration (Singleton)"""
-    list_display = [
-        'id',
-        'default_tokens_per_credit',
-        'default_credit_price_usd',
-        'credit_rounding_mode',
-        'enable_token_based_reporting',
-        'updated_at',
-        'updated_by'
-    ]
-    
-    fieldsets = (
-        ('Global Token-to-Credit Settings', {
-            'fields': ('default_tokens_per_credit', 'default_credit_price_usd', 'credit_rounding_mode'),
-            'description': 'These settings apply when no operation-specific config exists'
-        }),
-        ('Reporting Settings', {
-            'fields': ('enable_token_based_reporting',),
-            'description': 'Control token-based reporting features'
-        }),
-        ('Audit Trail', {
-            'fields': ('updated_by', 'updated_at'),
-            'classes': ('collapse',)
-        }),
-    )
-    
-    readonly_fields = ['updated_at']
-    
-    def has_add_permission(self, request):
-        """Only allow one instance (singleton)"""
-        from igny8_core.business.billing.models import BillingConfiguration
-        return not BillingConfiguration.objects.exists()
-    
-    def has_delete_permission(self, request, obj=None):
-        """Prevent deletion of the singleton"""
-        return False
-    
-    def save_model(self, request, obj, form, change):
-        """Track who made the change"""
-        obj.updated_by = request.user
-        super().save_model(request, obj, form, change)
-
-
 @admin.register(AIModelConfig)
 class AIModelConfigAdmin(SimpleHistoryAdmin, Igny8ModelAdmin):
     """