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/admin.py

@@ -19,21 +19,9 @@ class PlanAdmin(admin.ModelAdmin):
         ('Plan Info', {
             'fields': ('name', 'slug', 'price', 'billing_cycle', 'features', 'is_active')
         }),
-        ('User / Site Limits', {
+        ('Account Management Limits', {
             'fields': ('max_users', 'max_sites', 'max_industries', 'max_author_profiles')
         }),
-        ('Planner Limits', {
-            'fields': ('max_keywords', 'max_clusters', 'daily_cluster_limit', 'daily_keyword_import_limit', 'monthly_cluster_ai_credits')
-        }),
-        ('Writer Limits', {
-            'fields': ('daily_content_tasks', 'daily_ai_requests', 'monthly_word_count_limit', 'monthly_content_ai_credits')
-        }),
-        ('Image Limits', {
-            'fields': ('monthly_image_count', 'monthly_image_ai_credits', 'max_images_per_task', 'image_model_choices')
-        }),
-        ('AI Controls', {
-            'fields': ('daily_ai_request_limit', 'monthly_ai_credit_limit')
-        }),
         ('Billing & Credits', {
             'fields': ('included_credits', 'extra_credit_price', 'allow_credit_topup', 'auto_credit_topup_threshold', 'auto_credit_topup_amount', 'credits_per_month')
         }),