[FEATURE]: add invoice download for go subscription

[sunweijun-2025-8-18]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Description

Users need a way to download invoices or receipts for the OpenCode Go subscription ($5/$10 month).

Current Behavior

• No option found in the user console (opencode.ai/auth) to view billing history.
• No documentation on how to obtain payment receipts.
• No automated email receipts received upon subscription payment.

Expected Behavior

• A "Download Invoice" or "Billing History" section in the user console.
• Automated email receipts sent to the registered email address after each billing cycle.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• [FEATURE]: Zen dashboard - usage totals, per-model breakdown, and invoices #13497: Also requests invoice/billing history functionality as part of a dashboard feature
• [FEATURE]:建议出年费套餐，并且支持开发票 #20252: Requests invoice support for subscriptions (annual plan context)

Please review these issues and consider adding your input there if they cover the same need.