Defeat scan 429s with multi-backend rotation + cooldown

The "CORS" errors were 429 Too Many Requests (a 429 has no CORS headers, so
the browser reports it as a CORS failure). SSL/hosting is irrelevant.

Per-request retries made it worse by multiplying load on an already
rate-limited host. Replace with:
- round-robin across two Esplora-compatible backends (mempool.space +
  Blockstream), halving per-host load
- park a host on an 8s cooldown when it 429s instead of retrying it
- keep the global throttle (2 concurrent, 200ms gap)
- map Blockstream's /fee-estimates to the named fee tiers

Unit-tested rotation/cooldown; live scan of 40 addresses now completes in
~8s with no failures.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

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