feat: add notification for task status (#19965)

## Description

Send a notification to the workspace owner when an AI task’s app state
becomes `Working` or `Idle`.
An AI task is identified by a workspace build with `HasAITask = true`
and `AITaskSidebarAppID` matching the agent app’s ID.

## Changes

* Add `TemplateTaskWorking` notification template.
* Add `TemplateTaskIdle` notification template.
* Add `GetLatestWorkspaceAppStatusesByAppID` SQL query to get the
workspace app statuses ordered by latest first.
* Update `PATCH /workspaceagents/me/app-status` to enqueue:
  * `TemplateTaskWorking` when state transitions to `working`
  * `TemplateTaskIdle` when state transitions to `idle`
* Notification labels include:
  * `task`: task initial prompt
  * `workspace`: workspace name
* Notification dedupe: include a minute-bucketed timestamp (UTC
truncated to the minute) in the enqueue data to allow identical content
to resend within the same day (but not more than once per minute).

Closes: https://github.com/coder/coder/issues/19776

coderd/database/migrations/000376_task_status_notifications.down.sql

@@ -0,0 +1,4 @@
+-- Remove Task 'working' transition template notification
+DELETE FROM notification_templates WHERE id = 'bd4b7168-d05e-4e19-ad0f-3593b77aa90f';
+-- Remove Task 'idle' transition template notification
+DELETE FROM notification_templates WHERE id = 'd4a6271c-cced-4ed0-84ad-afd02a9c7799';

coderd/database/migrations/000376_task_status_notifications.up.sql

@@ -0,0 +1,63 @@
+-- Task transition to 'working' status
+INSERT INTO notification_templates (
+	id,
+	name,
+	title_template,
+	body_template,
+	actions,
+	"group",
+	method,
+	kind,
+	enabled_by_default
+) VALUES (
+			 'bd4b7168-d05e-4e19-ad0f-3593b77aa90f',
+			 'Task Working',
+			 E'Task ''{{.Labels.workspace}}'' is working',
+			 E'The task ''{{.Labels.task}}'' transitioned to a working state.',
+			 '[
+				 {
+					 "label": "View task",
+					 "url": "{{base_url}}/tasks/{{.UserUsername}}/{{.Labels.workspace}}"
+				 },
+				 {
+					 "label": "View workspace",
+					 "url": "{{base_url}}/@{{.UserUsername}}/{{.Labels.workspace}}"
+				 }
+			 ]'::jsonb,
+			 'Task Events',
+			 NULL,
+			 'system'::notification_template_kind,
+			 true
+		 );
+
+-- Task transition to 'idle' status
+INSERT INTO notification_templates (
+	id,
+	name,
+	title_template,
+	body_template,
+	actions,
+	"group",
+	method,
+	kind,
+	enabled_by_default
+) VALUES (
+			 'd4a6271c-cced-4ed0-84ad-afd02a9c7799',
+			 'Task Idle',
+			 E'Task ''{{.Labels.workspace}}'' is idle',
+			 E'The task ''{{.Labels.task}}'' is idle and ready for input.',
+			 '[
+				 {
+					 "label": "View task",
+					 "url": "{{base_url}}/tasks/{{.UserUsername}}/{{.Labels.workspace}}"
+				 },
+				 {
+					 "label": "View workspace",
+					 "url": "{{base_url}}/@{{.UserUsername}}/{{.Labels.workspace}}"
+				 }
+			 ]'::jsonb,
+			 'Task Events',
+			 NULL,
+			 'system'::notification_template_kind,
+			 true
+		 );