Phase 0: Foundation & Credit System - Initial implementation

- Updated CREDIT_COSTS constants to Phase 0 format with new operations
- Enhanced CreditService with get_credit_cost() method and operation_type support
- Created AccountModuleSettings model for module enable/disable functionality
- Added AccountModuleSettingsSerializer and ViewSet
- Registered module settings API endpoint: /api/v1/system/settings/account-modules/
- Maintained backward compatibility with existing credit system

backend/igny8_core/modules/system/urls.py

@@ -7,7 +7,7 @@ from .views import AIPromptViewSet, AuthorProfileViewSet, StrategyViewSet, syste
 from .integration_views import IntegrationSettingsViewSet
 from .settings_views import (
     SystemSettingsViewSet, AccountSettingsViewSet, UserSettingsViewSet,
-    ModuleSettingsViewSet, AISettingsViewSet
+    ModuleSettingsViewSet, AccountModuleSettingsViewSet, AISettingsViewSet
 )
 router = DefaultRouter()
 router.register(r'prompts', AIPromptViewSet, basename='prompts')
@@ -17,6 +17,7 @@ router.register(r'settings/system', SystemSettingsViewSet, basename='system-sett
 router.register(r'settings/account', AccountSettingsViewSet, basename='account-settings')
 router.register(r'settings/user', UserSettingsViewSet, basename='user-settings')
 router.register(r'settings/modules', ModuleSettingsViewSet, basename='module-settings')
+router.register(r'settings/account-modules', AccountModuleSettingsViewSet, basename='account-module-settings')
 router.register(r'settings/ai', AISettingsViewSet, basename='ai-settings')
 
 # Custom URL patterns for integration settings - matching reference plugin structure