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`.

> 🤖

docs/ai-coder/agents/platform-controls/chat-auto-archive.md

@@ -26,6 +26,15 @@ root regardless of individual pin status. Admins and users who want
 to retain a conversation long after its last message should pin the
 root.
 
+## Notifications
+
+When your chats are auto-archived, you receive a digest notification
+listing the titles of the archived conversations and the
+auto-archive window currently configured.
+
+If you find the digest noisy, you can disable the "Chats
+Auto-Archived" notification entirely from your notification preferences.
+
 ## Interaction with retention
 
 Auto-archive and deletion are two independent controls:
@@ -79,6 +88,4 @@ set `auto_archive_days` back to `0`.
 
 Each auto-archived root chat produces an audit log entry with the
 background subsystem tag `chat_auto_archive`. Cascaded children are
-not audited individually. The audit entry records the chat ID, owner
-ID, and organization ID, and the diff shows `archived` flipping from
-`false` to `true`.
+not audited individually.