Add unit tests for the backend pagination clamping, defaults, and meta math

[Baskarayelu]

Closes #990

Summary

• Add src/lib/backend/__tests__/pagination.test.ts with 13 focused test cases
• Cover default page/pageSize values and offset computation
• Cover clamping: page=0, negative page, pageSize above MAX_PAGE_SIZE, custom maxPageSize
• Cover non-numeric and float inputs falling back or throwing PaginationParseError
• Assert totalPages, hasNextPage, hasPrevPage meta math including zero-total edge case
• Verify correct data slicing for middle and last (partial) pages

Test plan

• Run pnpm test (requires node_modules installed) — all 13 tests in the new file pass
• No regressions against existing test suite

🤖 Generated with Claude Code

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