feat: warm configured price caches on startup

[jamilahmadzai]

Closes #25

Bounty: GrantFox escrow for issue #25

Summary

• Parse WATCHED_ASSETS into validated { code, issuer } entries with duplicate removal.
• Warm all configured assets through the existing price oracle before the HTTP server binds.
• Continue startup after a 30-second warm timeout and log success/failure counts.
• Document WATCHED_ASSETS in .env.example.

Validation

• npm ci --ignore-scripts --no-audit
• npm test -- test/cacheWarm.test.js test/config.test.js --runInBand (8 passing)
• npm test -- --runInBand (189 passing)
• git diff --check