Reduce worker concurrency from 4 to 2

Reduce idle read timeout from 120s to 60s

Add batching for IMAP metadata fetches

internal/config/config.go

@@ -25,7 +25,7 @@ func Load() (Config, error) {
 		AuthUser:          os.Getenv("AUTH_USER"),
 		AuthPass:          os.Getenv("AUTH_PASS"),
 		SessionSecret:     []byte(os.Getenv("SESSION_SECRET")),
-		WorkerConcurrency: 4,
+		WorkerConcurrency: 2,
 		PprofAddr:         os.Getenv("PPROF_ADDR"),
 	}
 	if v := os.Getenv("WORKER_CONCURRENCY"); v != "" {