Fix/182 developer pagination

[HereIsMuhammad]

Closes #182

Summary

Adds initial batching + progressive background loading for the developer
dataset instead of fetching everything (~1000 records) in one giant payload.

What changed

• GET /api/developers?limit=&offset= — new optional pagination, ordered
  by score desc. Without limit, behaves exactly as before (full array),
  so nothing else that hits this endpoint breaks.
• app/page.jsx now fetches a 500-record initial batch (fast TTI, loading
  overlay dismisses immediately), then quietly fetches the rest in 500-
  record background chunks, merging into state as each page arrives.
• Guards against clobbering an active search: if the user searches while
  background pages are still loading, the growing dataset won't overwrite
  their results.
• lib/pagination.js — extracted, unit-tested param parsing (clamping,
  defaults, backward-compat).

Testing

node --test tests/pagination.test.js — 8/8 passing.

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[HereIsMuhammad]

@sajeetharan
Hi! Have you saw that?