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/metrics/metrics.go

@@ -112,6 +112,7 @@ func (c *Collector) Collect(ch chan<- prometheus.Metric) {
 		health.StateUp,
 		health.StateDown,
 		health.StatePaused,
+		health.StateDisabled,
 		health.StateRemoved,
 	}
 
@@ -173,4 +174,3 @@ func Register(reg prometheus.Registerer, src StateSource) *Collector {
 	reg.MustRegister(TransitionTotal)
 	return coll
 }
-