Improve S3 URL Expiration Detection and Retry Logic

[luandro]

Closes #115

Fix to detect and handle expiring S3 URLs in fresh content fetched from Notion.

Changes

1. URL Expiration Check (imageReplacer.ts)

   • Added isUrlExpiringSoon(url, thresholdSeconds) utility
   • Checks S3 URLs for X-Amz-Expires + X-Amz-Date or Expires
   • Returns true if expiration is within threshold (default 5 mins)

2. Validation & Retry in Cache Loaders (cacheLoaders.ts)

   • Modified loadWithCache to accept a validateResult function
   • loadBlocksForPage: checking blocks JSON for expiring URLs
   • loadMarkdownForPage: checking markdown string for expiring URLs
   • If content has expiring URLs, it retries the fetch with linear backoff (up to 3 times)

3. Unit Tests

   • Created scripts/notion-fetch/__tests__/urlExpiration.test.ts to verify expiration logic
   • 6 tests for URL expiration detection
   • 47 tests for markdown processing with retry

Verification Results

• ✅ All 116 tests passing
• ✅ TypeScript type checking passes
• ✅ Pre-commit hook passes (eslint --max-warnings=0)
• ✅ All CI checks passing

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