Code quality batch

[luispabon]

Summary

• Add graceful SIGINT/SIGTERM shutdown and run the container under tini.
• Add flat ESLint config and wire lint scripts to run ESLint plus Prettier.
• Split the large app test file into focused suites with shared helpers and added coverage for validation, fetch errors, redirects, security, parsing, markdown, config, and concurrency.
• Move release publishing to package.json-backed vX.Y.Z tags, with Docker image publishing and GitHub Release creation in the tag workflow.
• Restrict Node test discovery to test/*.test.js so helper files are not executed as empty suites.
• Remove planning artifacts after successful review.

Review

Final review status: pass. No blocking or non-blocking findings remained.

Verification

• PNPM_CONFIG_MINIMUM_RELEASE_AGE=0 npx pnpm@11.7.0 lint
• PNPM_CONFIG_MINIMUM_RELEASE_AGE=0 npx pnpm@11.7.0 test
• docker build -t readability-js .
• Container SIGTERM shutdown probe, exit status 0

Residual Risk

• The GitHub release workflow metadata logic was simulated locally, but the workflow has not been exercised by pushing a real release tag in this review.

Closes #62
Closes #61
Closes #60
Closes #55