delivery-log-storage: the tailer keeps the lines of queued messages

Every mail.log line headed by a queue id is written to send_log_lines as the
tailer reads it, dated by the line's own stamp. The id is taken from the head
of the line only - the tail is the far end's text - and has to look like one:
"NOQUEUE:" and "warning:" stand in the same place.

The tailer does not check for a send-log row first: a message's first lines
come before the journal milter writes it. Lines that never get a row are
removed by the prune, which deletes lines older than an hour whose queue id
no row carries. It runs at start, after each send-log sweep and hourly.

While the tailer catches up at start a line is written only if it is not
there already, as the inbound journal does: its place is saved every few
seconds, so an unclean stop means reading some lines twice.

Checked on Windows: go vet clean; go test ./internal/logtail passes except
TestFollowTailsAndRotates, which fails there on renaming an open file and
always did. New tests: which line heads are queue ids, a line dated by its
stamp, no second copy after a rewound offset, the prune cadence and cutoff.

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

internal/logtail/retention_test.go

@@ -30,6 +30,14 @@ func (s *retentionProbeStore) DeleteSendLogBefore(cutoff time.Time) (int64, erro
 	return 0, nil
 }
 
+func (s *retentionProbeStore) InsertSendLogLine(store.SendLogLine) error { return nil }
+
+func (s *retentionProbeStore) InsertSendLogLineOnce(store.SendLogLine) (bool, error) {
+	return false, nil
+}
+
+func (s *retentionProbeStore) PruneSendLogLines(time.Time) (int64, error) { return 0, nil }
+
 func (s *retentionProbeStore) LogtailState(string) (store.LogtailState, bool, error) {
 	return store.LogtailState{}, false, nil
 }