[codex] Add price source circuit breakers

[vku2018]

Refs #11

Summary

• Add a reusable CircuitBreaker utility with closed/open/half-open states, configurable thresholds, cooldown, and INFO-level transition logging.
• Wrap each price source in priceOracle with an independent breaker so open circuits skip upstream calls and return null immediately.
• Expose current source circuit states from /health and document the circuit breaker environment defaults.
• Add focused Jest coverage for state transitions, health output, config defaults, and oracle-level open-circuit skip behavior.

Validation

• npm ci --ignore-scripts --no-audit
• npm test -- test/circuitBreaker.test.js test/config.test.js test/health.test.js test/resilience.test.js (20 passing)
• git diff --check

Full npm test was also attempted. It still fails on unrelated existing suites, including test/webhookSignature.test.js parse failure and pre-existing response-shape expectations in auth/rate-limit/prices/webhook/airdrop/requestId tests.