chore(smoke): tolerate indexer catching up + relax threshold to 500 blocks

[TuCopFi]

Summary

• Relaxes the pre-release backend smoke's indexer-health check from a hard 100-block ceiling to 500 blocks, and, if that's exceeded, gives the indexer 10 seconds to prove it's actively catching up (delta >= 50 blocks) before failing.
• Post-mortem from the 1.118.8 pre-release: indexer briefly hit 3876 blocks lag after an RPC hiccup, then caught up seconds later. The old threshold flagged this as a hard failure and blocked the release for no real backend issue.

Behavior

1. First sample: if lagBlocks <= 500, pass immediately.
2. Otherwise wait 10s and re-sample.
3. Pass if the second sample is either under 500, or dropped by at least 50 blocks (proof the indexer is draining backlog).
4. Fail only when the second sample is still above 500 AND is not catching up.

Test plan

• bash -n scripts/pre-release-backend-smoke.sh (syntax check)
• Full run against https://tucop-backend-production.up.railway.app (all 6 checks pass, indexer healthy at time of test)
• Next release cycle validates the retry branch during a real catch-up window