delivery-log-storage: the table for a message's log lines

Migration 0003_send_log_lines.sql, DDL only: queue id, the line's own time,
the line. No reference to send_log, because a message's first lines are
logged before the journal milter writes its row and one queue id is several
rows.

Store methods: InsertSendLogLine; InsertSendLogLineOnce for lines the tailer
reads twice after an unclean stop; SendLogLines, which gives a row the lines
of its queue id dated from an hour before the row to the queue lifetime after
it, so a later message under a reused id is not shown; PruneSendLogLines,
which deletes lines whose queue id has no send-log row - the whole of the
table's retention. A line is cut to 4096 bytes.

Nothing writes or reads the table yet. Checked on Windows: go vet and
go test ./internal/store pass, with the schema test pinning the new table,
its index and user_version 3.

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

docs/plans/delivery-log-storage.md

@@ -142,7 +142,7 @@ ordinary case.
 Target version cut: **`2.2.0`** (MINOR). One commit per step;
 [development.md](../development.md) § Plan checklists.
 
-- [ ] Migration `0003_send_log_lines.sql`, store methods (insert, insert once, lines of a row, prune) and their tests — **Opus**
+- [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**
 - [ ] 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**