fix(pagination): reject out-of-range feed pages

[rissrice2105-agent]

Summary

• return a real 404 when the requested feed page is past the final page
• keep page 1 valid when the feed is empty
• share the page-count calculation between feed validation and pagination UI
• add regression coverage for empty and partial pages

Fixes #78

Testing

• production reproduction: /page/999999 returns HTTP 200 before this fix
• 61 tests passed
• TypeScript typecheck passed
• git diff --check