fix(backend): validate configured model provider pair

backend/src/app.ts

@@ -23,7 +23,7 @@ export interface BuildAppDeps {
 }
 
 export function buildApp(config: AppConfig, deps?: BuildAppDeps): FastifyInstance {
-  const app = Fastify({ logger: false });
+  const app = Fastify({ logger: { level: "warn" }, disableRequestLogging: true });
 
   // Allow any origin in dev/e2e; tighten in production via config if needed.
   app.register(cors, {
@@ -45,7 +45,12 @@ export function buildApp(config: AppConfig, deps?: BuildAppDeps): FastifyInstanc
     Math.round(config.ollamaEnsureTimeoutMs / 1000),
   );
 
-  const listModels = deps?.listModels ?? createPiModelLister(config);
+  const listModels = deps?.listModels ?? createPiModelLister(config, {
+    warn: (detail) => app.log.warn(
+      { component: "pi-model-list", detail },
+      "Pi enabled-model configuration warning",
+    ),
+  });
   const getSettings = deps?.getSettings ?? (() => effectiveSettings(config, loadSettings(config)));
 
   const authenticate = authPreHandler(config.authMode);