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/plans/delivery-log-storage.md

@@ -146,7 +146,7 @@ Target version cut: **`2.2.0`** (MINOR). One commit per step;
 - [x] Log tailer: store queue-id lines, write once while catching up, prune with the send-log sweep and hourly — **Opus**
 - [x] The two texts, mockup first: the *Outbound log* help topic and the empty note (`design:` commit) — **Opus** (the help topic is in the mockups; the empty note is not drawn there - the `out-message` mockup shows a message with lines - and changes with the page)
 - [x] 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**
+- [x] [architecture.md](../architecture.md), [guide.md](../guide.md), [schema-migrations.md](../schema-migrations.md) — **Opus** (schema-migrations.md went in with the migration)
 - [ ] `go vet`, `go test`, e2e: a message's page still lists its lines after a forced rotation — **Opus**
 - [ ] Technical review — **Opus**
 - [ ] Security review — **Fable**