Implement circuit breaker for external price source calls

[prodbycorne]

Overview

When CoinGecko or CoinMarketCap are down or rate-limiting us, every price request waits for a full timeout before falling back. A circuit breaker will open after repeated failures and skip the failing source for a cool-down period, making responses faster and reducing load on recovering upstreams.

Behaviour

State	Description
Closed	Normal operation — requests pass through
Open	Source is failing — skip immediately, return null
Half-open	After cool-down, allow one test request; if it succeeds, close; if it fails, reopen

Configuration (per source)

CIRCUIT_BREAKER_FAILURE_THRESHOLD=3    # failures before opening
CIRCUIT_BREAKER_SUCCESS_THRESHOLD=1    # successes to close from half-open
CIRCUIT_BREAKER_TIMEOUT_MS=30000       # cool-down before half-open

Implementation

Create src/utils/circuitBreaker.js:

class CircuitBreaker {
  constructor(name, options) { ... }
  async call(fn) { ... }  // wraps the source fetch
  isOpen() { ... }
  recordSuccess() { ... }
  recordFailure() { ... }
}

Wrap each source in priceOracle.js with its own breaker instance.

Expose circuit state via GET /health:

{
  "circuits": {
    "coingecko": "closed",
    "coinmarketcap": "open",
    "stellar_dex": "closed"
  }
}

Acceptance Criteria

• CircuitBreaker class implemented with closed/open/half-open states
• Each price source has its own circuit breaker instance
• Open circuit returns null immediately (no network call)
• Circuit state visible in /health response
• State transitions logged at INFO level
• Unit tests for all three state transitions
• Config via env vars with documented defaults

[Achievers-sketch]

I can implement a resilient circuit breaker system for external price providers, adding failure thresholds, cooldown recovery, health visibility, logging, and tests.