WIP: add admin monitor snapshot endpoint for Admin UI

[nicepopo86-lang]

Summary

This is a small, scoped first deliverable toward #556 (Admin UI bounty): an admin-only monitoring API surface in Nutshell that a future UI can consume.

Added

• GET /v1/admin/monitor (admin-only)
  • DB counts: promises, proofs_used, proofs_pending, mint_quotes, melt_quotes
  • 24h request volume: mint/melt quote counts
  • host metrics: disk total/free, load average, process CPU seconds
• API key gating with MINT_ADMIN_API_KEY (X-Admin-Key header)
• typed response models for admin monitor payload
• unit tests for monitor snapshot builder + admin key validation
• docs/env updates for enabling the endpoint

Why this slice

Issue #556 asks for monitoring (DB entries, recent requests, host stats). This PR provides that backend primitive in-core so an external or embedded Admin UI can display it.

Checks

• python3 -m compileall cashu/mint/admin.py cashu/mint/router.py cashu/core/models.py cashu/core/settings.py tests/mint/test_admin_monitor.py

(Full pytest was not runnable in this environment because pytest is not installed.)

Refs #556