Proxy: handling invalid_grant refresh failures

Problem

When an OAuth refresh_token is revoked/expired, Google token refresh returns invalid_grant. Previously the proxy could repeatedly pick the same broken account, repeatedly fail refresh, and eventually return a 503 error due to an effectively unusable token pool.

Behavior after this change

1) Persistently disable the account on invalid_grant

• When token refresh fails with invalid_grant, the proxy marks that account as disabled on disk:
  • disabled: true
  • disabled_at: <unix timestamp>
  • disabled_reason: "invalid_grant: …" (truncated)
• The account is also removed from the in-memory token pool, preventing retry storms.

2) Skip disabled accounts when building the token pool

• During TokenManager::load_accounts, account JSON files with disabled: true are skipped.
• Reload clears the in-memory pool and re-reads the on-disk state so disables/enables take effect immediately.

3) Immediate reload when accounts change (if proxy is running)

Account mutations that affect proxy availability trigger a best-effort token pool reload when the proxy is running:

• Adding an account
• Completing OAuth login
• Updating tokens via the UI (account upsert)

Re-enabling an account

If a user updates credentials in the UI (token upsert) and changes either refresh_token or access_token, the account is automatically re-enabled by clearing:

• disabled
• disabled_reason
• disabled_at

This supports the workflow where a revoked token is replaced manually without requiring a proxy restart.

Data model / compatibility

Accounts gain three new fields:

• disabled (bool, default false)
• disabled_reason (string | null)
• disabled_at (number | null)

These fields are optional and use defaults, so existing account files continue to load.

Operational notes

• The disabled_reason is truncated to avoid bloating the account JSON.
• No secrets are intentionally written into disabled_reason; it is derived from the refresh error string.
• If desired, the UI can surface these fields to explain why an account is no longer used by the proxy.

Testing (suggested)

• Reproduce: force an account to have a revoked/invalid refresh_token and trigger a proxy request that requires refresh.
• Expected:
  • Proxy logs show the invalid_grant failure and account disable.
  • The account is removed from the token pool and will not be selected again.
  • After updating the token via UI, the account is re-enabled and becomes eligible without restarting the proxy.