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>

CHANGELOG.md

@@ -11,8 +11,14 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
   The table that will hold what Postfix wrote about a message for as long as
   its send-log row lives, with the store methods to write a line, to write it
   once, to read the lines of a row and to prune
-  ([plans/delivery-log-storage.md](docs/plans/delivery-log-storage.md)). Nothing
-  writes or reads it yet.
+  ([plans/delivery-log-storage.md](docs/plans/delivery-log-storage.md)).
+- **The log tailer keeps what Postfix writes about queued messages.** Every
+  `mail.log` line headed by a queue id is stored with the line's own time, as
+  the tailer reads it; after an unclean stop a line read twice is stored once.
+  Lines whose queue id has no send-log row - the row was pruned, or the
+  message never had one (inbound relay mail, bounces) - are deleted once they
+  are an hour old, with every send-log sweep and hourly between. No page reads
+  them yet.
 
 ## [2.1.0] - 2026-10-12