Add pagination to posts list

[wassertim]

Problem

All posts matching current filters are fetched and rendered in a single response. The listPosts() query in src/db.ts has no LIMIT clause, and the home route renders every result at once. This won't scale as post count grows.

Proposed Solution

Add cursor-based or offset/limit pagination to the posts list:

• Add LIMIT and OFFSET to listPosts() queries in src/db.ts
• Return pagination metadata (total count, current page, has more)
• Add "Load more" or page navigation controls using HTMX
• Apply pagination to all sort/filter/search modes

Acceptance Criteria

• Posts are loaded in fixed-size pages (e.g. 20 per page)
• User can load additional pages via UI controls
• Pagination works with all existing filters (status, sort, search)
• No-JS fallback works (progressive enhancement)