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.

test/support/nextcloud.js

@@ -50,10 +50,11 @@ const listing = (folder) => {
  * stand-in's documents during a test, and `stop()`.
  */
 export async function startApi(env = {}) {
-  const stats = { requests: 0, connections: 0, listings: 0 };
+  const stats = { requests: 0, connections: 0, listings: 0, userAgent: null };
   let down = false;
   const source = createServer((req, res) => {
     stats.requests++;
+    stats.userAgent = req.headers["user-agent"] || null;
     if (req.method === "PROPFIND") stats.listings++;
     if (down) return res.writeHead(503).end();
     const path = decodeURIComponent(req.url);