Refresh customer-facing Lambda README

[rferreira]

Summary

• Rewrite the scanii-lambda README around the customer-facing AWS Serverless Application Repository deployment flow.
• Add a prominent Deploy on AWS section, document parameters, required post-deploy S3 trigger setup, result actions, and verification steps.
• Remove the hard-coded endpoint list in favor of the maintained endpoint documentation.
• Add inline comments for advanced configuration defaults in lib/config.js.
• Correct SAM action defaults to the accepted true/false values and bump the SAR package version to 3.0.2.

Tests

• npm test: PASS, 34 passing, 4 pending because scanii-cli was not reachable locally.
• git diff --check: PASS.
• package/package-lock version check: PASS.
• sam validate: not run because SAM CLI is not installed in this environment.

Notes

• The README still links to the existing SAR app name UvaSoftware-Scanii-Lambda because release.yml still publishes to that application ID.