Phase 0: Remove plan operation limit fields (credit-only system)

- Removed all operation limit fields from Plan model
- Kept account management limits (max_users, max_sites, etc.)
- Updated PlanSerializer to remove limit fields
- Updated PlanAdmin to remove limit fieldsets
- Created migration to remove limit fields from database
- Plan model now only has credits, billing, and account management fields

backend/igny8_core/auth/serializers.py

@@ -11,10 +11,10 @@ class PlanSerializer(serializers.ModelSerializer):
         model = Plan
         fields = [
             'id', 'name', 'slug', 'price', 'billing_cycle', 'features', 'is_active',
-            'max_users', 'max_sites', 'max_keywords', 'max_clusters', 'max_content_ideas',
-            'monthly_word_count_limit', 'monthly_ai_credit_limit', 'monthly_image_count',
-            'daily_content_tasks', 'daily_ai_request_limit', 'daily_image_generation_limit',
-            'included_credits', 'image_model_choices', 'credits_per_month'
+            'max_users', 'max_sites', 'max_industries', 'max_author_profiles',
+            'included_credits', 'extra_credit_price', 'allow_credit_topup',
+            'auto_credit_topup_threshold', 'auto_credit_topup_amount',
+            'stripe_product_id', 'stripe_price_id', 'credits_per_month'
         ]