Multi-strategy approach to find readable versions of paywalled articles:
1. Wayback Machine (check existing + Save Page Now)
2. Google Web Cache
3. archive.ph (read-only check for existing snapshots)
Adds archiveUrl field to Note model, /api/articles/unlock endpoint,
unlock button on note detail page, and browser extension integration.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
