fix: bootstrap user preferences without invalidating DWH cache

backend/src/app.ts

@@ -64,7 +64,13 @@ export function buildApp(config: AppConfig, deps?: BuildAppDeps): FastifyInstanc
     // The real runner persists preferences through the harness repository. The file fallback
     // only keeps older isolated route tests and externally injected runners compatible.
     if (typeof runner.preferencesGet === "function") {
-      return effectiveSettings(config, await runner.preferencesGet());
+      const stored = await runner.preferencesGet() as Settings;
+      if (Object.keys(stored).length === 0) {
+        const seeded = effectiveSettings(config, loadSettings(config));
+        await runner.preferencesSet(seeded);
+        return seeded;
+      }
+      return effectiveSettings(config, stored);
     }
     return effectiveSettings(config, loadSettings(config));
   };