Reduce worker concurrency from 4 to 2

Reduce idle read timeout from 120s to 60s

Add batching for IMAP metadata fetches

internal/config/config_test.go

@@ -31,7 +31,7 @@ func TestLoadDefaults(t *testing.T) {
 	if cfg.HTTPAddr != ":8080" {
 		t.Errorf("HTTPAddr = %q, want :8080", cfg.HTTPAddr)
 	}
-	if cfg.WorkerConcurrency != 4 {
-		t.Errorf("WorkerConcurrency = %d, want 4", cfg.WorkerConcurrency)
+	if cfg.WorkerConcurrency != 2 {
+		t.Errorf("WorkerConcurrency = %d, want 2", cfg.WorkerConcurrency)
 	}
 }