fix(billing): honor table sort order in per-user lazy load

[karpikpl]

Fixes #418

Summary

• apply v-data-table sort options before selecting lazy-loaded per-user billing rows
• forward billing sort/page options to the per-user billing endpoint for DB-backed spend ordering
• add regression coverage for token and spend sort lazy-load requests

Validation

• npm test -- tests/billing-per-user-lazy-load.spec.ts tests/billing-credit-reader.spec.ts tests/billing-credits.db-first.spec.ts --run
• npm run build

Note: a full npm test -- --run in the symlinked /tmp worktree failed before running Nuxt-environment suites because @nuxt/test-utils tried to import /@fs/home/.../node_modules/@nuxt/test-utils/dist/runtime/entry.mjs; targeted node suites passed and the production build succeeded.