feat(coderd): notify on task completion/failure (#20327)

Adds notifications on task transitions to completed or failure state.

Authored by Claude, I reviewed it and it appears to be legit.

coderd/database/migrations/000383_add_task_completed_failed_notification_templates.down.sql

@@ -0,0 +1,5 @@
+-- Remove Task 'completed' transition template notification
+DELETE FROM notification_templates WHERE id = '8c5a4d12-9f7e-4b3a-a1c8-6e4f2d9b5a7c';
+
+-- Remove Task 'failed' transition template notification
+DELETE FROM notification_templates WHERE id = '3b7e8f1a-4c2d-49a6-b5e9-7f3a1c8d6b4e';

coderd/database/migrations/000383_add_task_completed_failed_notification_templates.up.sql

@@ -0,0 +1,63 @@
+-- Task transition to 'complete' status
+INSERT INTO notification_templates (
+	id,
+	name,
+	title_template,
+	body_template,
+	actions,
+	"group",
+	method,
+	kind,
+	enabled_by_default
+) VALUES (
+			 '8c5a4d12-9f7e-4b3a-a1c8-6e4f2d9b5a7c',
+			 'Task Completed',
+			 E'Task ''{{.Labels.workspace}}'' completed',
+			 E'The task ''{{.Labels.task}}'' has completed successfully.',
+			 '[
+				 {
+					 "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 'failed' status
+INSERT INTO notification_templates (
+	id,
+	name,
+	title_template,
+	body_template,
+	actions,
+	"group",
+	method,
+	kind,
+	enabled_by_default
+) VALUES (
+			 '3b7e8f1a-4c2d-49a6-b5e9-7f3a1c8d6b4e',
+			 'Task Failed',
+			 E'Task ''{{.Labels.workspace}}'' failed',
+			 E'The task ''{{.Labels.task}}'' has failed. Check the logs for more details.',
+			 '[
+				 {
+					 "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
+		 );