delivery-log-storage: a message's page reads its stored lines

The Delivery log box on a message's page was filled by searching the current
mail.log on every view; that file is rotated daily, so the box was empty for
anything not sent today. deliveryLog now asks the store for the row's lines -
its queue id, dated within the row's queue lifetime plus a day
(RetryPolicy.LongestLifetime) - and shows the latest 200, oldest first.

When nothing is stored the page still searches the current file, which is
what shows a message accepted before 2.2.0. When that finds nothing either,
the note no longer blames rotation for the ordinary case: it says lines are
kept with the row since 2.2.0. The Outbound log help topic in help.go follows
the mockup changed in fc184ec, word for word.

Checked on Windows: go vet clean; go test ./internal/web/... ./internal/postfix
./internal/store pass, the guards among them. New test: lines shown from the
table when mail.log holds none of them, without another queue id's lines and
without those of an earlier message under the same id.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/postfix/retry.go

@@ -129,6 +129,16 @@ func (p RetryPolicy) BackoffCap() string {
 	return FormatDuration(p.MaximalBackoff)
 }
 
+// LongestLifetime is how long Postfix may go on working on a message: the
+// larger of the queue lifetime for mail and for the bounces it generates. It
+// bounds which stored log lines can be about a send-log row (store.SendLogLines).
+func (p RetryPolicy) LongestLifetime() time.Duration {
+	if p.BounceQueueLifetime > p.MaximalQueueLifetime {
+		return p.BounceQueueLifetime
+	}
+	return p.MaximalQueueLifetime
+}
+
 // QueueLifetime is the human string for maximal_queue_lifetime.
 func (p RetryPolicy) QueueLifetime() string {
 	return FormatDuration(p.MaximalQueueLifetime)