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>

docs/plans/delivery-log-storage.md

@@ -143,7 +143,7 @@ Target version cut: **`2.2.0`** (MINOR). One commit per step;
 [development.md](../development.md) § Plan checklists.
 
 - [x] Migration `0003_send_log_lines.sql`, store methods (insert, insert once, lines of a row, prune) and their tests — **Opus**
-- [ ] Log tailer: store queue-id lines, write once while catching up, prune with the send-log sweep and hourly — **Opus**
+- [x] Log tailer: store queue-id lines, write once while catching up, prune with the send-log sweep and hourly — **Opus**
 - [ ] The two texts, mockup first: the *Outbound log* help topic and the empty note (`design:` commit) — **Opus**
 - [ ] Message page reads the table, falls back to the current `mail.log`; help text in `help.go` — **Opus**
 - [ ] [architecture.md](../architecture.md), [guide.md](../guide.md), [schema-migrations.md](../schema-migrations.md) — **Opus**