[codex] Add price source retry backoff

[vku2018]

Refs #23

Summary

• Add fetchWithRetry for price-source requests with retryable network/429/5xx handling, non-retryable 400/401/403 handling, DEBUG retry logging, and Retry-After support.
• Route CoinGecko, CoinMarketCap, and Stellar DEX order-book calls through the retry helper.
• Add PRICE_SOURCE_RETRY_COUNT config with documented default 3.
• Add focused unit coverage for retry success, max failure, 401 no-retry, retry-after parsing, and CMC config expectations.

Validation

• npm test -- test/fetchWithRetry.test.js test/coinmarketcap.test.js test/config.test.js (15 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.