nats: say whether a user's grant is in force on the running server, not only written (hq issue 353)

nats_user_can read the user list on the machine. A list written is not a list the bus enforces until
the server reloads it: on 2026-10-09 the controller asked between the write and the reload's effect.
The answer now carries in_force — allowed in the file, and the server's config_load_time after the
file's write — with why when it is not, and anything not known is not in force.

modules/nats/cmd/nats-tools/monitor.go

@@ -65,3 +65,24 @@ func ReadUsers(ctx context.Context) ([]byte, error) {
 	}
 	return out, nil
 }
+
+// UsersWritten is when the composed user list the server includes was last written, read in the container
+// to the nanosecond (`stat -c %y`): what the server's last reload must come after for the list to be in force.
+func UsersWritten(ctx context.Context) (time.Time, error) {
+	container := env("MESH_NATS_CONTAINER", "mesh-broker-nats")
+	path := env("MESH_NATS_USERS_FILE", "/etc/nats/accounts.conf")
+	out, err := exec.CommandContext(ctx, "docker", "exec", container, "stat", "-c", "%y", path).Output()
+	if err != nil {
+		return time.Time{}, fmt.Errorf("when the bus's user list was written could not be read inside %s: %w", container, err)
+	}
+	return ParseWritten(string(out))
+}
+
+// ParseWritten reads `stat -c %y`: "2026-10-09 17:38:13.422211788 +0000".
+func ParseWritten(s string) (time.Time, error) {
+	t, err := time.Parse("2006-01-02 15:04:05.999999999 -0700", strings.TrimSpace(s))
+	if err != nil {
+		return time.Time{}, fmt.Errorf("when the bus's user list was written is not a time: %q", strings.TrimSpace(s))
+	}
+	return t, nil
+}