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/ai_vision.py

@@ -22,6 +22,7 @@ from google.genai import types
 
 from app.config import settings
 from app.services.storage_backend import get_storage_backend
+from app.services.runtime_config import runtime_ai_config
 
 logger = logging.getLogger(__name__)
 
@@ -128,7 +129,7 @@ async def _generate_gemini(
         contents.append(types.Part.from_bytes(data=image_bytes, mime_type=media_type))
     contents.append(prompt)
 
-    model = model_override or settings.GEMINI_MODEL
+    model = model_override or runtime_ai_config.model or settings.GEMINI_MODEL
 
     async def _call():
         return await asyncio.wait_for(
@@ -165,7 +166,7 @@ async def _generate_openrouter(
         logger.warning("ai.openrouter.no_key")
         return {"text": None, "usage": (None, None)}
 
-    model = model_override or settings.OPENROUTER_MODEL
+    model = model_override or runtime_ai_config.model or settings.OPENROUTER_MODEL
 
     headers = {
         "Authorization": f"Bearer {settings.OPENROUTER_API_KEY}",
@@ -233,7 +234,7 @@ async def _generate(
     model_override: Optional[str] = None,
 ) -> dict:
     """Dispatcher vers le bon provider, avec override optionnel par client."""
-    provider = provider_override or settings.AI_PROVIDER.lower()
+    provider = provider_override or runtime_ai_config.provider or settings.AI_PROVIDER.lower()
     logger.info("ai.generate", extra={"provider": provider})
 
     if provider == "openrouter":
@@ -289,8 +290,8 @@ async def analyze_image(
     if not settings.AI_ENABLED:
         return {}
 
-    active_provider = provider_override or settings.AI_PROVIDER
-    active_model = model_override or (
+    active_provider = provider_override or runtime_ai_config.provider or settings.AI_PROVIDER
+    active_model = model_override or runtime_ai_config.model or (
         settings.OPENROUTER_MODEL if active_provider == "openrouter"
         else settings.GEMINI_MODEL
     )