feat: chat auto-archive owner digest notifications (#24643)

Depends on #24642

Adds per-owner digest notifications onto the chat auto-archive
subsystem.

Each tick's archived rows are grouped by owner, the top 25 titles per
owner are rendered into a new `Chats Auto-Archived` notification
template, and any remainder surfaces as `and N more`. Each digest is
per-tick, so users with large amounts of purgeable data may get multiple
notifications in sequence (one per user per tick).

The template body branches on `retention_days`: when retention is
disabled (`retention_days=0`), users are told archived chats are kept
indefinitely rather than falsely claiming imminent deletion.

### Changes
- migration `000XXX_chat_auto_archive_notification_template` adds new
notification template
- `dbpurge`: threads `notifications.Enqueuer` through `New`; and
enqueues notification message.
- `cli/server.go`: passes `options.NotificationsEnqueuer` into
`dbpurge.New`.
- `coderd/notifications/events.go`: new `TemplateChatAutoArchiveDigest`
UUID.
- `coderd/inboxnotifications.go`: inbox registration.
- Docs: adds a `Notifications` section to `chat-auto-archive.md`.

> 🤖

coderd/database/queries/chats.sql

@@ -1482,4 +1482,6 @@ SELECT
     )::timestamptz AS last_activity_at
 FROM archived a
 LEFT JOIN to_archive t ON t.id = a.id
+-- created_at ASC flows through to dbpurge's digest truncation; see
+-- buildDigestData in dbpurge.go for the tradeoff rationale.
 ORDER BY (a.root_chat_id IS NULL) DESC, a.owner_id ASC, a.created_at ASC, a.id ASC;