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_test.go

@@ -333,6 +333,7 @@ func TestStateString(t *testing.T) {
 		{StateUp, "up"},
 		{StateDown, "down"},
 		{StatePaused, "paused"},
+		{StateDisabled, "disabled"},
 		{StateRemoved, "removed"},
 	}
 	for _, c := range cases {