feat: add balance top-up requests and admin credit API

[romy651]

Summary

When balance limits are enabled, users can exhaust their credits before the next automatic refill, but they currently have no built-in way to ask an administrator for help. Administrators also lack a capability-gated, audited, and retry-safe API for inspecting balances and applying a manual reset.

This PR adds a user balance top-up request flow in Settings and in the chat balance notice, exposes pending-request state through the balance API, and adds administrative endpoints for listing user balances and applying credits. Administrative credits are atomic and idempotent, request resolution is lease-protected, and successful operations are recorded in the audit log.

The existing general announcement banner is unchanged.

How it works

User submits request (Settings or chat balance notice)
  -> POST /api/balance/request
  -> one pending BalanceRequest per user
  -> admin lists balances/pending requests
  -> POST /api/admin/balance/:userId
  -> idempotent credit + audit entry + request resolution

The admin UI is provided by LibreChat-AI/admin-panel#149.

Type of change

• Feature
• Tests / tooling / CI

Testing

Tested environments/configuration:

• MongoDB standalone and replica-set-backed test paths
• Balance enabled with and without automatic refill
• Production client build

Automated tests:

• npm run build:packages
• npm run build:client
• TypeScript checks for client, API, data-provider, and data-schemas
• Upstream-relative static checks, including ESLint, Prettier, import sorting, design rules, circular dependencies, config migrations, and unused i18n keys
• 47 balance/admin API tests
• 100 balance-request and transaction tests
• 60 server-controller tests
• 14 client balance-notice and request-dialog tests

Screenshots / recordings

State	Light mode	Dark mode
Running low
Out of credits

Checklist

• I reviewed my own changes
• Relevant tests have been added or updated
• Existing relevant tests pass
• The change does not introduce new warnings or errors
• User-facing or complex behavior is documented where necessary
• Required dependency changes have been merged/published
• Required documentation PR: N/A