fix: Extend cache TTL on GitHub PR fetch failure to prevent retry storms

Address PR #688 review feedback from CodeRabbit: When a GitHub PR fetch
fails and we return stale cached data, also update the fetchedAt timestamp.
This prevents the original TTL from expiring and causing every subsequent
poll to retry the failing request, which would still hammer GitHub during
API outages.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/server/src/routes/worktree/routes/list.ts

@@ -255,6 +255,8 @@ async function fetchGitHubPRs(
     // repeated API calls during GitHub API flakiness or temporary outages
     if (cached) {
       logger.warn(`Failed to fetch GitHub PRs, returning stale cache: ${getErrorMessage(error)}`);
+      // Extend cache TTL to avoid repeated retries during outages
+      githubPRCache.set(projectPath, { prs: cached.prs, fetchedAt: Date.now() });
       return cached.prs;
     }
     // No cache available, log warning and return empty map