feat: validate environment configuration

[leo1987820]

Summary

• add envalid-based startup validation for runtime configuration
• switch Redis configuration to a validated REDIS_URL and expose DATABASE_URL as a production-required setting
• update .env.example and README with types, defaults, and required production vars
• add Jest coverage for production validation failures and NODE_ENV=test defaults

Closes #21

Verification

• npm test -- config.test.js --runInBand
• npm test -- --runInBand

[prodbycorne]

fix branch conflicts

[leo1987820]

Updated the branch on top of current main and resolved the conflict with the new Redis graceful-degradation changes by keeping the lazy connection/offline-queue behavior while using the validated REDIS_URL.

Verification:

• npm.cmd test -- config.test.js --runInBand
• npm.cmd test -- --runInBand