feat: enhance user-agent identification and implement backoff strategy for university source requests

- Added PSU_CONTACT environment variable to docker-compose for owner contact in User-Agent.
- Updated source.js to include user-agent construction with service version and contact information.
- Implemented exponential backoff strategy for failed requests to the university, with retry logic based on HTTP response status.
- Modified status.js to display backoff information in the status response.
- Adjusted watch.js to reduce request frequency during university downtime.
- Enhanced test coverage with new backoff.test.js to validate backoff behavior and user-agent format.

server/status/status.js

@@ -102,7 +102,9 @@ function tiles(d) {
       "Университет",
       failing ? "bad" : "ok",
       failing ? "Не отвечает" : "Отвечает",
-      failing ? `с ${dateTime(failing)}` : d.watch?.lastError || `таблица недель: ${d.calendar?.weeksFiles?.length ? "есть" : "нет"}`,
+      failing
+        ? `с ${dateTime(failing)}` + (d.catalog?.backoff?.retryAt ? ` · следующая попытка ${dateTime(d.catalog.backoff.retryAt)}` : "")
+        : d.watch?.lastError || `таблица недель: ${d.calendar?.weeksFiles?.length ? "есть" : "нет"}`,
     ),
     tile(
       "Разбор",