Live AI provider/model switching without restart

Backend:
- RuntimeAIConfig singleton: mutable in-memory overrides for provider/model
- PATCH /admin/api/ai/config: change active provider/model on the fly
- GET /admin/api/ai/config: read current runtime state
- ai_vision.py: uses runtime_ai_config before falling back to settings
- health/status endpoints reflect runtime overrides

Frontend:
- 'Définir comme actif' button in AI config section
- Live feedback message on save

app/services/runtime_config.py

@@ -0,0 +1,63 @@
+"""
+Runtime AI Config — mutable overrides pour le provider/model actif.
+
+Permet à l'admin de changer le fournisseur/modèle sans redémarrer.
+Les overrides sont stockés en mémoire (perdus au redémarrage du backend).
+"""
+import threading
+from typing import Optional
+
+
+class RuntimeAIConfig:
+    """Singleton thread-safe pour les overrides AI runtime."""
+
+    def __init__(self):
+        self._lock = threading.Lock()
+        self._provider: Optional[str] = None
+        self._model: Optional[str] = None
+
+    @property
+    def provider(self) -> Optional[str]:
+        with self._lock:
+            return self._provider
+
+    @property
+    def model(self) -> Optional[str]:
+        with self._lock:
+            return self._model
+
+    @property
+    def has_override(self) -> bool:
+        with self._lock:
+            return self._provider is not None
+
+    def set(self, provider: Optional[str] = None, model: Optional[str] = None) -> None:
+        """Définit les overrides. None = utiliser le défaut de settings."""
+        with self._lock:
+            self._provider = provider
+            self._model = model
+
+    def clear(self) -> None:
+        """Réinitialise aux valeurs par défaut."""
+        with self._lock:
+            self._provider = None
+            self._model = None
+
+    def to_dict(self) -> dict:
+        """Retourne l'état actuel pour les API."""
+        from app.config import settings
+        with self._lock:
+            return {
+                "provider": self._provider or settings.AI_PROVIDER,
+                "model": self._model or (
+                    settings.OPENROUTER_MODEL if (self._provider or settings.AI_PROVIDER) == "openrouter"
+                    else settings.GEMINI_MODEL
+                ),
+                "is_overridden": self._provider is not None,
+                "override_provider": self._provider,
+                "override_model": self._model,
+            }
+
+
+# Singleton global
+runtime_ai_config = RuntimeAIConfig()