[Evo Studio] Add account balance, contribution credits, and cloud training billing

[pearlq12345]

This PR builds on feat/evo-train-vla-backend and adds the minimum account and billing layer for Evo Studio.

Changes:

• Add account ledger with training balance and contribution points
• Add top-up order and payment receiver config APIs
• Add dataset upload reward points
• Freeze estimated training balance before cloud training starts
• Settle training cost after completion with service fee support
• Protect admin billing routes with EVO_STUDIO_ADMIN_TOKEN

Notes:

• balanceCents is cash-like training balance.
• creditPoints is non-cash contribution credit for uploaded datasets.
• Real payment credentials are not stored in code.
• The current file-backed ledger is for development/internal testing; production should use a database backend.