Upstash command limit hit: free tier 500K/month and budget-cap rate limiting

# Upstash: hitting the command limit (free tier 500K/month, budget caps)

## What you see

- Requests start failing or slowing once the free tiers 500K monthly
  commands are exhausted.
- On pay-as-you-go with a budget cap, the database is rate limited
  past the cap. You get email warnings at 70% and 90% first.
- /multi-exec batches get discarded when a command would exceed the
daily request limit.

## Confirm

Check the console usage graphs for the database. Command count near
500K on a free database, or near your budget cap on a paid one,
confirms it. This is not a code bug.

## Fix

1. Short term: upgrade to pay as you go ($0.2 per 100K commands) or
   raise the budget cap.
2. Cut volume: batch with /pipeline or Promise.all auto-pipelining,
   lengthen poll intervals, pick cheaper rate-limit algorithms
   (fixed window over sliding window).
3. Set or lower a budget cap so the next spike rate limits instead of
   surprising you on the invoice.

Remember: the free 500K is not free on top of paid tiers. Upgrading
meters everything from the first command.

## Verify

Usage graph flattens or the tier change reflects in the console, and
requests succeed again.