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`.

> 🤖

cli/server.go

@@ -1074,7 +1074,7 @@ func (r *RootCmd) Server(newAPI func(context.Context, *coderd.Options) (*coderd.
 			defer shutdownConns()
 
 			// Ensures that old database entries are cleaned up over time!
-			purger := dbpurge.New(ctx, logger.Named("dbpurge"), options.Database, options.DeploymentValues, options.PrometheusRegistry, &coderAPI.Auditor)
+			purger := dbpurge.New(ctx, logger.Named("dbpurge"), options.Database, options.DeploymentValues, options.PrometheusRegistry, &coderAPI.Auditor, dbpurge.WithNotificationsEnqueuer(options.NotificationsEnqueuer))
 			defer purger.Close()
 
 			// Updates workspace usage