log: write durations as text, not nanoseconds (closes #228)

The JSON log wrote a Go duration as a bare count of nanoseconds, so
the watcher starting line showed dnsInterval 120000000000 for 2m and a
delivery retry showed retryIn 1015437050. Each duration logged is now
passed through its String() form: dnsInterval and tlsInterval when the
watcher starts, retryIn on a delivery retry, and latency on a
succeeded port check. A test checks that retryIn is logged as the
text of the wait the retry actually took.

The request log's latency_ms is left as it is: its key names its
unit.

Model: opus-5-5

internal/portcheck/portcheck.go

@@ -193,7 +193,9 @@ func (c *Checker) checkConnection(
 	c.log.Debug(
 		"port check succeeded",
 		"target", target,
-		"latency", latency,
+		// As text: the JSON log writes a time.Duration as bare
+		// nanoseconds.
+		"latency", latency.String(),
 	)
 
 	return &PortResult{