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/billing/constants.py

@@ -1,22 +1,25 @@
 """
-Credit Cost Constants
+Credit Cost Constants - Phase 0: Credit-Only System
+All features are unlimited. Only credits restrict usage.
 """
 CREDIT_COSTS = {
-    'clustering': {
-        'base': 1,  # 1 credit per 30 keywords
-        'per_keyword': 1 / 30,
-    },
-    'ideas': {
-        'base': 1,  # 1 credit per idea
-    },
-    'content': {
-        'base': 3,  # 3 credits per full blog post
-    },
-    'images': {
-        'base': 1,  # 1 credit per image
-    },
-    'reparse': {
-        'base': 1,  # 1 credit per reparse
-    },
+    # Existing operations
+    'clustering': 10,                    # Per clustering request
+    'idea_generation': 15,              # Per cluster → ideas request
+    'content_generation': 1,             # Per 100 words
+    'image_prompt_extraction': 2,       # Per content piece
+    'image_generation': 5,              # Per image
+    
+    # Legacy operation names (for backward compatibility)
+    'ideas': 15,                        # Alias for idea_generation
+    'content': 1,                       # Alias for content_generation (per 100 words)
+    'images': 5,                        # Alias for image_generation
+    'reparse': 2,                       # Alias for image_prompt_extraction
+    
+    # NEW: Phase 2+ operations
+    'linking': 8,                       # Per content piece (NEW)
+    'optimization': 1,                  # Per 200 words (NEW)
+    'site_structure_generation': 50,    # Per site blueprint (NEW)
+    'site_page_generation': 20,         # Per page (NEW)
 }