Reduce worker concurrency from 4 to 2

Reduce idle read timeout from 120s to 60s

Add batching for IMAP metadata fetches

internal/imapx/batch_test.go

@@ -0,0 +1,54 @@
+package imapx
+
+import (
+	"testing"
+
+	"github.com/emersion/go-imap/v2"
+)
+
+// metaBatches must tile the sequence 1..total into contiguous, non-overlapping
+// windows of at most batchSize, covering every message exactly once. A single
+// unbounded FETCH 1:* is what wedges large mailboxes; batching keeps each
+// command short so the server stays responsive and ctx can be checked between
+// windows.
+func TestMetaBatches(t *testing.T) {
+	cases := []struct {
+		total, size uint32
+		want        []imap.SeqRange
+	}{
+		{0, 1000, nil},
+		{1, 1000, []imap.SeqRange{{Start: 1, Stop: 1}}},
+		{1000, 1000, []imap.SeqRange{{Start: 1, Stop: 1000}}},
+		{1001, 1000, []imap.SeqRange{{Start: 1, Stop: 1000}, {Start: 1001, Stop: 1001}}},
+		{2500, 1000, []imap.SeqRange{{Start: 1, Stop: 1000}, {Start: 1001, Stop: 2000}, {Start: 2001, Stop: 2500}}},
+	}
+	for _, c := range cases {
+		got := metaBatches(c.total, c.size)
+		if len(got) != len(c.want) {
+			t.Fatalf("total=%d size=%d: got %d windows %v, want %d %v", c.total, c.size, len(got), got, len(c.want), c.want)
+		}
+		for i := range got {
+			if got[i] != c.want[i] {
+				t.Fatalf("total=%d size=%d window %d: got %+v want %+v", c.total, c.size, i, got[i], c.want[i])
+			}
+		}
+	}
+}
+
+// Every window must be within 1..total and the windows must be gap-free so no
+// message is skipped or fetched twice.
+func TestMetaBatchesCoverage(t *testing.T) {
+	const total, size = 4321, 1000
+	got := metaBatches(total, size)
+	if got[0].Start != 1 {
+		t.Fatalf("first window must start at 1, got %d", got[0].Start)
+	}
+	if last := got[len(got)-1]; last.Stop != total {
+		t.Fatalf("last window must stop at total=%d, got %d", total, last.Stop)
+	}
+	for i := 1; i < len(got); i++ {
+		if got[i].Start != got[i-1].Stop+1 {
+			t.Fatalf("gap/overlap between window %d (%+v) and %d (%+v)", i-1, got[i-1], i, got[i])
+		}
+	}
+}