Persist one browser user agent for Axios and WebSockets

lib/db.ts

@@ -389,6 +389,18 @@ export function setSetting(key: string, value: string): void {
     db.prepare(`INSERT INTO settings (key, value) VALUES (?, ?) ON CONFLICT(key) DO UPDATE SET value = excluded.value`).run(key, value);
 }
 
+/** Seed a generated setting once, including across simultaneous server workers.
+ *  Acquire the write lock before reading so they cannot choose different values. */
+export function getOrCreateSetting(key: string, create: () => string): string {
+    return db.transaction(() => {
+        const saved = getSetting(key);
+        if (saved?.trim()) return saved;
+        const value = create();
+        setSetting(key, value);
+        return value;
+    }).immediate();
+}
+
 // ── Instruments ──────────────────────────────────────────────────────────────
 
 const SYMBOLS = ['NQ','MNQ','ES','MES','YM','MYM','RTY','M2K','GC','MGC','SI','CL','MCL','NG','ZB','ZN','ZF','6E','6J','6B'];