Reduce worker concurrency from 4 to 2

Reduce idle read timeout from 120s to 60s

Add batching for IMAP metadata fetches

internal/imapx/conn.go

@@ -11,7 +11,7 @@ import (
 // live transfer never trips it — only a genuinely dead/mute socket does. This
 // is what stops an account from wedging in "running" forever when a server
 // accepts a command (e.g. a large FETCH) and then goes silent.
-var idleReadTimeout = 120 * time.Second
+var idleReadTimeout = 60 * time.Second
 
 // idleConn wraps a net.Conn and arms a fresh read deadline before every Read.
 // It lives beneath any TLS layer (the raw TCP conn), so the deadline governs