Dashboard: connect to real billing data via API endpoints

[Wilbert957]

Problem

The dashboard at GET /dashboard currently shows hardcoded mock data. It is not connected to any live backend.

Proposed Solution

1. Add admin API endpoints (no EIP-191 auth, use DAYTONA_ADMIN_KEY header)

Endpoint	Data
GET /admin/status	active sandbox count, next voucher flush ETA, compute spend/hr
GET /admin/sandboxes	list of all sandboxes with owner, status, accrued neuron
GET /admin/activity	recent events (voucher settled, sandbox created/stopped, deposits)
GET /admin/contract	contract address, beacon, impl, provider, TEE signer, pricing params

2. Update web/dashboard.html

• Replace hardcoded values with fetch() calls to the above endpoints
• Poll every 10–30 seconds for live refresh
• Show loading/error states

3. Auth

Admin endpoints protected by X-Admin-Key: <DAYTONA_ADMIN_KEY> header check (same key already used for Daytona).

Acceptance Criteria

• /admin/status returns JSON with live data from Redis
• /admin/sandboxes returns active sandbox list
• /admin/activity returns last N events
• Dashboard auto-refreshes without page reload
• All admin endpoints return 401 without valid key