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/watcher/watcher.go

@@ -123,8 +123,10 @@ func (w *Watcher) Run(ctx context.Context) {
 		"watcher starting",
 		"domains", len(w.config.Domains),
 		"hostnames", len(w.config.Hostnames),
-		"dnsInterval", w.config.DNSInterval,
-		"tlsInterval", w.config.TLSInterval,
+		// As text: the JSON log writes a time.Duration as bare
+		// nanoseconds.
+		"dnsInterval", w.config.DNSInterval.String(),
+		"tlsInterval", w.config.TLSInterval.String(),
 	)
 
 	w.RunOnce(ctx)