feat(ai): selection fournisseur/modele par defaut + guide architecture

- Persistance ai_default_provider / ai_default_models dans data/config.json
- Lecture + rechargement a chaud dans backend/ai.py (get_default_provider, reload_ai_config)
- UI: selecteurs Fournisseur/Modele par defaut dans la section Cles API IA (i18n FR/EN)
- docs/AI_ARCHITECTURE_GUIDE.md: architecture cible, catalogue d'outils, decisions MCP

backend/main.py

@@ -4286,6 +4286,8 @@ _DEFAULT_CONFIG = {
     "prefix_max_expansions": 50,
     "recent_files_limit": 20,
     "max_backups_per_file": 10,
+    "ai_default_provider": "deepseek",
+    "ai_default_models": {},
 }
 
 
@@ -4341,6 +4343,12 @@ async def api_set_config(body: dict = Body(...), current_user=Depends(require_ad
                     detail=f"Invalid type for '{key}': expected {expected_type.__name__}, got {type(value).__name__}",
                 )
     _save_config(current)
+    if any(k.startswith("ai_") for k in updated_keys):
+        try:
+            from backend.ai import reload_ai_config
+            reload_ai_config()
+        except Exception as e:
+            logger.warning(f"Failed to reload AI config: {e}")
     logger.info(f"Config updated: {updated_keys}")
     return current