make async database integrity check

backend/src/workers/db-verify.js

@@ -0,0 +1,18 @@
+const { parentPort, workerData } = require('worker_threads');
+const Database = require('better-sqlite3');
+
+if (!parentPort) throw new Error("Must be run in a worker thread");
+
+try {
+  const { filePath } = workerData;
+  const db = new Database(filePath, { readonly: true, fileMustExist: true });
+  
+  // This is the CPU-heavy operation
+  const result = db.prepare("PRAGMA integrity_check;").get();
+  
+  db.close();
+  parentPort.postMessage(result.integrity_check === "ok");
+} catch (error) {
+  // Any error means invalid or corrupt DB
+  parentPort.postMessage(false);
+}