fix: paginate blog sitemap to include all posts

[miohtama]

Why

The blog sitemap (/blog/sitemap.xml) only listed 100 posts even though 129 exist. The route requested limit=all from Ghost, but Ghost's Content API silently caps a single response at 100 posts ("limit":100,"pages":2 in the pagination metadata). Because the sitemap did a single request with no pagination, every post beyond the first 100 was dropped from search-engine indexing, and the gap grows as more posts are published.

Lessons learnt

• Ghost Content API's limit=all is not honoured as "everything in one response" — it is capped at 100 per page. To fetch a complete collection you must paginate and follow meta.pagination.next until it is null.
• Verify sitemap counts against Ghost's meta.pagination.total, not the number of <loc> entries returned by a single call — the two can silently diverge at the 100 boundary.
• The integration suite fetches the blog sitemap against the live Ghost API (no mock), so pagination logic is better locked in with a deterministic unit test that stubs fetch.

Summary

• Add getAllPosts() to src/lib/blog/client.ts — pages through Ghost with the maximum page size (100), following meta.pagination.next until exhausted, with a guard against a non-advancing next to avoid infinite loops.
• Use getAllPosts() in src/routes/blog/sitemap.xml/+server.ts instead of the single getPosts({ limit: 'all' }) call.
• Add src/lib/blog/client.test.ts covering multi-page (>100), max page size, and single-page cases.
• Changelog entry.