Add retry logic with exponential backoff for price source HTTP calls

[prodbycorne]

Overview

All three price sources (coingecko.js, coinmarketcap.js, stellarDex.js) perform a single HTTP request with no retry. Transient network errors silently return null prices and cause anomaly false-positives. Retry with exponential backoff will dramatically improve data availability.

Retry Policy

Attempt	Delay	When to retry
1	—	Initial call
2	500 ms	On network error or 429/5xx
3	2 s	—
4	8 s	—

Non-retryable: 400, 401, 403 — fail immediately (invalid API key, bad request).

Implementation

Create src/utils/fetchWithRetry.js:

async function fetchWithRetry(url, options = {}, retries = 3, baseDelayMs = 500) {
  for (let attempt = 0; attempt <= retries; attempt++) {
    try {
      const res = await axios.get(url, { timeout: 10_000, ...options });
      return res;
    } catch (err) {
      const status = err.response?.status;
      const isRetryable = !status || status === 429 || status >= 500;
      if (!isRetryable || attempt === retries) throw err;
      const delay = baseDelayMs * Math.pow(2, attempt);
      await sleep(delay);
    }
  }
}

Replace bare axios.get calls in each source file with fetchWithRetry.

For 429 responses, honour the Retry-After header if present.

Acceptance Criteria

• fetchWithRetry utility created
• All three source files use it
• 429 responses respect Retry-After header when present
• 400/401/403 errors do NOT retry
• Retry attempts logged at DEBUG level with attempt number
• Unit tests: success on retry-2, permanent failure after max retries, 401 no-retry
• PRICE_SOURCE_RETRY_COUNT env var to override default (default: 3)