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/main.py

@@ -188,15 +188,20 @@ async def root():
 
 @app.get("/health", tags=["Santé"])
 async def health():
+    from app.services.runtime_config import runtime_ai_config
+    active_provider = runtime_ai_config.provider or settings.AI_PROVIDER
+    active_model = runtime_ai_config.model or (
+        settings.OPENROUTER_MODEL if active_provider == "openrouter"
+        else settings.GEMINI_MODEL
+    )
     ai_configured = (
-        (settings.AI_PROVIDER == "gemini" and bool(settings.GEMINI_API_KEY)) or
-        (settings.AI_PROVIDER == "openrouter" and bool(settings.OPENROUTER_API_KEY))
+        (active_provider == "gemini" and bool(settings.GEMINI_API_KEY)) or
+        (active_provider == "openrouter" and bool(settings.OPENROUTER_API_KEY))
     )
-    active_model = settings.OPENROUTER_MODEL if settings.AI_PROVIDER == "openrouter" else settings.GEMINI_MODEL
     return {
         "status": "healthy",
         "ai_enabled": settings.AI_ENABLED,
-        "ai_provider": settings.AI_PROVIDER,
+        "ai_provider": active_provider,
         "ai_configured": ai_configured,
         "ocr_enabled": settings.OCR_ENABLED,
         "model": active_model,