refactor: Shared Pagination Utility Extractor (#199)

[AnnabelJoe]

Extract the limit, order, and cursor query parameter handling into a single reusable pagination utility to eliminate repetition and ensure consistent behaviour across all paginated endpoints.

Changes:

• Created src/utils/pagination.js with parsePaginationParams
• Updated src/routes/account.js, src/routes/asset.js, and src/routes/transactions.js
• No behavior changes for valid inputs; invalid inputs now consistently return 400 ValidationError

Closes #199