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/migrations/000480_chat_auto_archive_notification_template.down.sql

@@ -0,0 +1 @@
+DELETE FROM notification_templates WHERE id = '764031be-4863-4220-867b-6ce1a1b7a5f5';

coderd/database/migrations/000480_chat_auto_archive_notification_template.up.sql

@@ -0,0 +1,34 @@
+-- Template for the per-owner chat auto-archive notification. Enqueue is
+-- per-tick (see dbpurge.dispatchChatAutoArchive): owners whose backlog
+-- spans multiple ticks receive multiple notifications, and
+-- notification_messages dedupe does not collapse them because each
+-- tick's payload differs. Users who find this noisy can disable the
+-- template from their notification preferences. The SMTP/webhook
+-- wrappers prepend "Hi {{.UserName}},", so body_template must not.
+INSERT INTO notification_templates (
+    id,
+    name,
+    title_template,
+    body_template,
+    actions,
+    "group",
+    method,
+    kind,
+    enabled_by_default
+)
+VALUES (
+    '764031be-4863-4220-867b-6ce1a1b7a5f5',
+    'Chats Auto-Archived',
+    E'Chats auto-archived after {{.Data.auto_archive_days}} days of inactivity',
+    E'The following chats were automatically archived:\n\n{{range .Data.archived_chats}}* "{{.title}}" (last active {{.last_activity_humanized}})\n{{end}}{{with .Data.additional_archived_count}}\n...and {{.}} more.\n\n{{end}}\n{{if eq .Data.retention_days "0"}}You can restore any of them from the Agents page; archived chats are kept indefinitely.{{else}}You can restore any of them from the Agents page within {{.Data.retention_days}} days, after which they will be permanently deleted.{{end}}',
+    '[
+        {
+            "label": "View chats",
+            "url": "{{base_url}}/agents?archived=archived"
+        }
+    ]'::jsonb,
+    'Chat Events',
+    NULL,
+    'system'::notification_template_kind,
+    true
+);