Resilience, rankings & billing polish
- Rankings: public leaderboard of the most-used models (last 30 days, anonymized aggregates).
- Mid-stream failover: if a provider closes the stream before emitting anything, the request retries on the next provider automatically.
- Price sync is now automatic — an hourly cron keeps per-token billing prices fresh (admins can still sync manually from Ops).
- Optional top-up fee (TOPUP_FEE_PERCENT) with the credited amount shown transparently before payment.