fix(web): use stored API key when fetching models for saved providers (#2910)

When editing an existing model, the edit form initializes apiKey as
empty for security. This caused "Fetch Available Models" to reject with
"please enter API Key first" even though the key is saved server-side.

Add model_index support: the frontend passes the model's index to the
backend, which looks up the stored key from config. The key never leaves
the backend. Provider and API base are validated to prevent a stored key
from being sent to an unrelated endpoint.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/frontend/src/api/models.ts

@@ -167,6 +167,7 @@ export interface FetchModelsRequest {
   provider: string
   api_key?: string
   api_base?: string
+  model_index?: number
 }
 
 export interface FetchModelsResponse {