fix(proxy): clear token-manager refresh state on account removal

Account removal only called usageCache.stopPolling(accountId), leaving
refreshFailures/backoffCounters/refreshInFlight entries in token-manager.ts
to linger until the 5-minute TTL sweep or 1000-entry cap. Same gap pattern
as #382, bounded rather than unbounded. clearAccountRefreshCache is already
called on reauth and usage-refresh paths — now also called on removal.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>