fix(billing): rank Top spenders server-side across all users

[karpikpl]

Fixes #417

Summary

• Add a DB-backed /api/billing-credits-top-users endpoint with default top 10 and configurable limit/metric query params.
• Rank top spenders with a single SUM(...) GROUP BY username query across all ingested billing rows.
• Update the Billing tab chart to consume the server-side global ranking instead of lazy-loaded table rows.

Validation

• Added failing regression coverage first for global top-N behavior.
• npm test -- tests/billing-credit-reader.spec.ts tests/billing-credits.db-first.spec.ts
• npm run build

Note: full npm test currently fails in this symlinked worktree on Nuxt test-utils resolving /@fs/... before any tests run; targeted non-Nuxt regression tests pass.