fix: resolve all failing tests and type errors

[stooit]

Summary

• Implement paginate() utility in packages/shared/src/utils/pagination.ts — was a stub throwing "not implemented"
• Fix User type field name in packages/shared/src/types.ts — changed userName to username to match test expectations
• Fix auth middleware case-sensitivity in packages/api/src/middleware/auth.ts — HTTP method comparison now case-insensitive so POST routes correctly bypass auth
• Fix missing helper in users route in packages/api/src/routes/users.ts — added inline badRequest response helper
• Add bun-types to tsconfig — enables proper type resolution for bun:test and process globals

Verification

All 22 tests pass (bun test) and tsc --noEmit exits cleanly with zero errors.

Assumptions

• The bun-types package was already installed in node_modules (no new dependency added)
• The process global declaration was removed from auth.ts in favour of the tsconfig types fix
• badRequest was implemented inline as a simple 400 JSON response helper since no shared utility existed