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.sql.go

@@ -5419,6 +5419,8 @@ type AutoArchiveInactiveChatsRow struct {
 // Archives inactive root chats (pinned and already-archived chats skipped),
 // cascading to children via root_chat_id. Limits apply to roots, not total
 // rows. Used by dbpurge.
+// created_at ASC flows through to dbpurge's digest truncation; see
+// buildDigestData in dbpurge.go for the tradeoff rationale.
 func (q *sqlQuerier) AutoArchiveInactiveChats(ctx context.Context, arg AutoArchiveInactiveChatsParams) ([]AutoArchiveInactiveChatsRow, error) {
 	rows, err := q.db.QueryContext(ctx, autoArchiveInactiveChats, arg.ArchiveCutoff, arg.LimitCount)
 	if err != nil {