fix(backend): harden provider credential isolation

backend/src/pi/list-models.ts

@@ -1,6 +1,7 @@
 import { spawn as nodeSpawn, type ChildProcessWithoutNullStreams } from "node:child_process";
 import type { AppConfig } from "../config.js";
 import { RpcClient } from "../rpc/rpc-client.js";
+import { buildPiChildEnv } from "./provider-credentials.js";
 
 export interface PiModel {
   provider: string;
@@ -34,7 +35,10 @@ export function createPiModelLister(cfg: AppConfig, opts: Opts = {}): () => Prom
   return async function listModels(): Promise<PiModel[]> {
     if (cache && now() - cache.at < ttlMs) return cache.models;
 
-    const env: NodeJS.ProcessEnv = { ...process.env };
+    const env = buildPiChildEnv({
+      provider: cfg.defaults.provider,
+      credentialFile: cfg.modelApiKeyFile,
+    });
     delete env.THT_DATA_ROOT;
     if (cfg.dataRoot !== undefined) env.THT_DATA_ROOT = cfg.dataRoot;
     const child = spawnFn(cfg.piBin, ["--mode", "rpc"], { cwd: cfg.harnessDir, env });