Flag guests that no backup job covers

The guest table marks with "!" each guest that Proxmox's not-backed-up
list returns. The frame's bottom border shows the count, e.g.
"mainframe · 2 without backup job". This uses Proxmox's own answer, so
it also counts jobs that select guests by pool.

If the coverage call fails, the guests are still listed and a warning
says the coverage is unknown.

Closes #9

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_cli.py

@@ -985,3 +985,21 @@ def test_apply_ip_rejects_netmask_form_and_zero_prefix(cidr):
     family = 6 if ":" in cidr else 4
     with pytest.raises(ValueError):
         cli.apply_ip({}, family, "static", cidr)
+
+
+# --- #9: guests not covered by any backup job -----------------------------------------
+
+
+def test_not_backed_up_returns_vmids(fake):
+    fake.result = (
+        json.dumps([{"vmid": 104, "name": "debian", "type": "lxc"}, {"vmid": "110"}]),
+        "",
+        0,
+    )
+    assert cli.not_backed_up() == {104, 110}
+    assert fake.argv == ["cv4pve-cli", "get", "cluster", "not-backed-up", "-o", "json"]
+
+
+def test_not_backed_up_empty(fake):
+    fake.result = ("[]", "", 0)
+    assert cli.not_backed_up() == set()