Refresh the repository tab counts while the page is open

Why

The open counts beside the Issues and Changes tabs only moved when the page was loaded, so a reader watching a repository had to reload to learn that new work had arrived.

What changed

• The repository header polls GET /r/{name}/counts every 30 seconds while the page is visible. The response is the two badges as out-of-band updates, so the current tab, the clone menu, a search field and any half-filled form stay where the reader left them.
• RepoCounts reads the same openCounts the page did and answers 404 for a repository the viewer cannot read, like every other route below /r/{name}.
• A badge whose number moved keeps one short ring (app.js compares the counts after each out-of-band swap), because a number changing elsewhere on the screen is easy to miss. The reduced-motion rule still applies.
• The count markup moved into two small templates (repo_issue_count, repo_request_count) so the page and the poll cannot drift apart on the element ids the swap matches.

Tests

• TestRepoTabCountsPollOutOfBand (server): the page carries the poll and both badges, the response is exactly two out-of-band badges, and an unknown repository answers 404.
• TestRepositoryPagesShareTheTabHeader and TestRepoTabsCountOpenIssuesAndRequests updated for the badge ids.
• vole workflow check and vole workflow run verify both pass.

Closes #141