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/ai.py

@@ -18,6 +18,7 @@ ProviderName = Literal["deepseek", "openrouter", "gemini", "ollama", "nvidia", "
 
 # Provider configurations — keys loaded from file or .env
 AI_KEYS_FILE = Path("data/api_keys.json")
+APP_CONFIG_FILE = Path(__file__).resolve().parent.parent / "data" / "config.json"
 
 def _read_ai_keys() -> dict:
     if not AI_KEYS_FILE.exists():
@@ -27,6 +28,16 @@ def _read_ai_keys() -> dict:
     except Exception:
         return {}
 
+
+def _read_app_config() -> dict:
+    """Read the persisted application config (``data/config.json``)."""
+    if not APP_CONFIG_FILE.exists():
+        return {}
+    try:
+        return json.loads(APP_CONFIG_FILE.read_text(encoding="utf-8"))
+    except Exception:
+        return {}
+
 def get_ai_key(env_name: str) -> str:
     """Get AI key: stored file first, then .env fallback."""
     keys = _read_ai_keys()
@@ -36,7 +47,7 @@ def get_ai_key(env_name: str) -> str:
 
 def _load_provider_keys():
     """Load AI keys from stored file, falling back to .env."""
-    return {
+    providers = {
         "deepseek": {
             "api_key": get_ai_key("DEEPSEEK_API_KEY"),
             "base_url": "https://api.deepseek.com/v1",
@@ -90,17 +101,46 @@ def _load_provider_keys():
             "auth_header": "Bearer {api_key}",
         },
     }
+    # Apply persisted per-provider model overrides (data/config.json).
+    overrides = _read_app_config().get("ai_default_models") or {}
+    if isinstance(overrides, dict):
+        for name, model in overrides.items():
+            if name in providers and isinstance(model, str) and model:
+                providers[name]["model"] = model
+    return providers
+
 
 PROVIDERS = _load_provider_keys()
 
 DEFAULT_PROVIDER: ProviderName = os.getenv("AI_DEFAULT_PROVIDER", "deepseek")  # type: ignore
 
 
+def get_default_provider() -> str:
+    """Resolve the default provider: ``data/config.json`` > env > ``deepseek``."""
+    provider = _read_app_config().get("ai_default_provider") or os.getenv("AI_DEFAULT_PROVIDER", "deepseek")
+    return provider if provider in PROVIDERS else "deepseek"
+
+
+def reload_ai_config() -> str:
+    """Reload provider keys and model overrides from disk into ``PROVIDERS`` in place.
+
+    Mutating ``PROVIDERS`` (rather than rebinding it) keeps references held by
+    other modules valid. Returns the resolved default provider.
+    """
+    global DEFAULT_PROVIDER
+    PROVIDERS.clear()
+    PROVIDERS.update(_load_provider_keys())
+    DEFAULT_PROVIDER = get_default_provider()  # type: ignore[assignment]
+    logger.info(f"AI config reloaded (default provider: {DEFAULT_PROVIDER})")
+    return DEFAULT_PROVIDER
+
+
 def _get_provider_config(provider: ProviderName | None = None) -> dict:
     """Get provider config, falling back to default if requested provider unavailable."""
-    p = provider or DEFAULT_PROVIDER
+    default = get_default_provider()
+    p = provider or default
     if p not in PROVIDERS:
-        p = DEFAULT_PROVIDER
+        p = default
     cfg = PROVIDERS[p]
     if not cfg["api_key"]:
         # Try next available provider