Distinguish disabled from removed backend state; add make fixstyle

Add StateDisabled for operator-initiated disable, keeping StateRemoved
for backends that disappear during a config reload. Previously both
used StateRemoved, which was confusing: "removed" implies the backend
no longer exists in config, but a disabled backend is still present
and can be re-enabled on the fly.

- health: add StateDisabled with String() "disabled", Disable() method
  with probe code "disabled". Record() rejects probes in all three
  inactive states (paused, disabled, removed).
- checker: DisableBackend calls backend.Disable() instead of Remove().
- docs: healthchecks.md rewritten for pause (goroutine cancelled, not
  just results discarded), and separate disabled/removed state rows.
  user-guide.md updated to match.
- Makefile: add fixstyle target (gofmt -w .).

internal/grpcapi/server_test.go

@@ -268,8 +268,8 @@ func TestEnableDisableBackend(t *testing.T) {
 	if err != nil {
 		t.Fatalf("DisableBackend: %v", err)
 	}
-	if info.State != "removed" {
-		t.Errorf("after disable: got %q, want removed", info.State)
+	if info.State != "disabled" {
+		t.Errorf("after disable: got %q, want disabled", info.State)
 	}
 	if info.Enabled {
 		t.Error("after disable: Enabled should be false")