fix: repair CI build & lint regressions on main

- openai_gateway_model_availability.go: pass platform through to
  listSchedulableAccounts so OpenAI/Grok diagnosis scopes to the
  correct candidate pool (build break introduced by Grok subscription
  PR #3310).
- no_account_error.go: drop redundant context.Context type on
  ctx := context.Background() to satisfy staticcheck ST1023.

backend/internal/handler/no_account_error.go

@@ -101,7 +101,7 @@ func classifyNoAccountErrorFromGin(
 	displayModel string,
 	platform string,
 ) noAccountErrorClassification {
-	var ctx context.Context = context.Background()
+	ctx := context.Background()
 	if c != nil && c.Request != nil {
 		ctx = c.Request.Context()
 	}