delivery-log-storage: architecture and guide

architecture.md § Log tailer gains "Stored lines": what is kept and why the
queue id is read from the head of a line, why the tailer does not check for a
row before writing, the one prune rule that is the table's retention, how a
row is given its lines when Postfix reuses queue ids, the write-once rule
while catching up, and the size. QueueLines is described as what it is now,
the fallback for rows without stored lines. guide.md: the message page keeps
its lines after rotation; retention also bounds the kept lines, which are in
a full backup.

Docs only.

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

docs/roadmap.md

@@ -36,7 +36,7 @@ in `git log` and [CHANGELOG.md](../CHANGELOG.md).
 | structured-logging | `log/slog` with levels and fields | candidate | — | — |
 | review-2026-08-followups | Remaining findings of the 2026-08-19 code review | candidate | — | — |
 | panel-notifications | E-mail notifications about important events | candidate | — | — |
-| delivery-log-storage | Keep a message's `mail.log` lines with its send-log row | in progress | 4/8 | [plans/delivery-log-storage.md](plans/delivery-log-storage.md) |
+| delivery-log-storage | Keep a message's `mail.log` lines with its send-log row | in progress | 5/8 | [plans/delivery-log-storage.md](plans/delivery-log-storage.md) |
 | password-reset | Password reset by e-mail | candidate | — | — |
 | phone-desktop-mode | A phone browser in "desktop site" mode gets the phone navigation | candidate | — | — |
 | panel-redesign | Panel redesign (accepted mockups, design contract) | shipped `2.0.0` | 27/27 | [plans/panel-redesign.md](plans/panel-redesign.md) |