email: real-time IMAP IDLE watchers (push on new mail); cron kept as backstop

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/servers/api/email/resync.ts

@@ -271,7 +271,19 @@ type ResyncParams = {
   userId: number;
 };
 
-export async function performResync({ accountId, userEmail, userId }: ResyncParams): Promise<ResyncResult> {
+// Coalesce concurrent resyncs of the same account within this process (IDLE + cron + manual can all
+// fire) — a caller arriving mid-resync just awaits the one already running.
+const resyncInFlight = new Map<number, Promise<ResyncResult>>();
+
+export function performResync(params: ResyncParams): Promise<ResyncResult> {
+  const running = resyncInFlight.get(params.accountId);
+  if (running) return running;
+  const p = doResync(params).finally(() => resyncInFlight.delete(params.accountId));
+  resyncInFlight.set(params.accountId, p);
+  return p;
+}
+
+async function doResync({ accountId, userEmail, userId }: ResyncParams): Promise<ResyncResult> {
   const account = await getEmailAccount(accountId);
   if (!account || account.userId !== userId) throw new Error('Account not found');