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/health/state.go

@@ -29,11 +29,12 @@ type ProbeResult struct {
 type State int
 
 const (
-	StateUnknown State = iota // initial state before first probe
-	StateUp
-	StateDown
-	StatePaused
-	StateRemoved // backend was removed from configuration
+	StateUnknown  State = iota // initial state before first probe
+	StateUp                    // backend is healthy
+	StateDown                  // backend has failed enough probes
+	StatePaused                // operator paused health checking
+	StateDisabled              // operator disabled the backend
+	StateRemoved               // backend removed from configuration by reload
 )
 
 func (s State) String() string {
@@ -46,6 +47,8 @@ func (s State) String() string {
 		return "down"
 	case StatePaused:
 		return "paused"
+	case StateDisabled:
+		return "disabled"
 	case StateRemoved:
 		return "removed"
 	default:
@@ -123,7 +126,7 @@ func New(name string, addr net.IP, rise, fall int) *Backend {
 // failure means the backend is not yet confirmed reachable), and to StateUp
 // once the counter reaches Rise consecutive passes.
 func (b *Backend) Record(r ProbeResult, maxHistory int) bool {
-	if b.State == StatePaused || b.State == StateRemoved {
+	if b.State == StatePaused || b.State == StateDisabled || b.State == StateRemoved {
 		return false
 	}
 	if r.OK {
@@ -196,6 +199,13 @@ func (b *Backend) Start(maxHistory int) Transition {
 	return b.Transitions[0]
 }
 
+// Disable transitions the backend to StateDisabled. Returns the transition.
+// After this call no further probe results are accepted.
+func (b *Backend) Disable(maxHistory int) Transition {
+	b.transition(StateDisabled, ProbeResult{Code: "disabled"}, maxHistory)
+	return b.Transitions[0]
+}
+
 // Remove transitions the backend to StateRemoved. Returns the transition.
 // After this call no further probe results are accepted.
 func (b *Backend) Remove(maxHistory int) Transition {