[Hitesh-XS]

Closes #22

What i did here:

Paginates the leaderboard so contributors ranked #51+ are reachable.

Changes

• Added LEADERBOARD_PAGE_SIZE = 50 (explore's PAGE_SIZE is untouched).
• Ordering is now ("-total", "pk") so ranking is stable across pages.
• Hero card and top-3 highlight bars appear on page 1 only; rank numbers continue across pages (page 2 starts at 51).
• Pagination links below the list, adapted from _issue_list.html (without the HTMX attributes, since this is a full-page view). ?country= is preserved in every page link.
• Caching: the leaderboard now caches the computed rows instead of the full rendered HTML, so the template renders per request. This removes the leaderboard's share of Logged-in users see the login button on Explore/Leaderboard (page cache ignores auth) #38 (anonymous navbar served to logged-in users). Explore still caches full pages and is not changed here.
• Added test_leaderboard_pagination covering page 1 (hero, order), page 2 (no hero, rank offset), bad page numbers, and country filter persistence in page links.

Testing

pytest core/tests.py: 28 passed.

[Ymax27]

Merged 🎉 Thanks a lot @Hitesh-XS, this is a really solid first PR: stable ordering, rank offset, country filter kept in the links, safe page parsing and a thorough test. And thanks for switching the leaderboard to row-level caching, that fixes the leaderboard half of #38 👏
Tiny non-blocking nits for a future PR if you feel like it: podium/rest/rest_offset are no longer used by the template, and two files are missing a trailing newline.

— Max