AI Vision: multi-provider config, model testing, per-client AI preferences

Backend:
- Add ai_provider + ai_model columns to APIClient model
- New admin_ai router: GET /admin/api/ai/status + POST /admin/api/ai/test
- Update ai_vision.py: provider_override + model_override support
- Update pipeline: load client AI prefs for image processing
- ClientUpdate/ClientResponse schemas include ai_provider/ai_model

Frontend:
- New AIConfigSection component: provider list, model dropdowns,
  test button with live results (no page refresh needed)
- AI types (ai.types.ts) + API (api/ai.ts)
- APIClient types extended with ai_provider/ai_model

app/schemas/auth.py

@@ -47,6 +47,8 @@ class ClientUpdate(BaseModel):
     scopes: Optional[List[str]] = None
     plan: Optional[ClientPlan] = None
     is_active: Optional[bool] = None
+    ai_provider: Optional[str] = Field(None, description="Fournisseur AI préféré (openrouter, gemini)")
+    ai_model: Optional[str] = Field(None, description="Modèle AI préféré")
 
 
 # ─────────────────────────────────────────────────────────────
@@ -62,6 +64,8 @@ class ClientResponse(BaseModel):
     scopes: List[str]
     plan: ClientPlan
     is_active: bool
+    ai_provider: Optional[str] = None
+    ai_model: Optional[str] = None
     created_at: Optional[datetime] = None
     updated_at: Optional[datetime] = None