fix: circuit break non-retryable price source failures

[jamilahmadzai]

Fixes #95

Bounty: GrantFox OSS campaign for issue #95

Summary

• add oracle-level source circuit state for errors tagged nonRetryable, with configurable cooldown and reminder logging
• stop retrying circuit-open price sources on every refresh cycle, retry after cooldown, and close the circuit after a successful fetch
• expose price_source_circuits in /health and document PRICE_SOURCE_CIRCUIT_COOLDOWN_MS / PRICE_SOURCE_CIRCUIT_REMINDER_MS
• tag CoinGecko 401/403 responses as nonRetryable so it follows the same permanent-auth-failure path as CoinMarketCap

Validation

• npm test -- test/resilience.test.js test/config.test.js test/coinmarketcap.test.js test/coingecko.test.js --runInBand
• npm test -- --runInBand (23 suites, 190 tests)
• node --check src/services/priceOracle.js && node --check src/services/sources/coingecko.js && node --check src/index.js && node --check src/config.js
• git diff --check