fix: backend audit, security, tests and CI

[th4RUK4]

Summary

• fix duplicate token sequence generation
• add API input validation and request size limits
• add automated backend tests
• add GitHub Actions CI
• add admin JWT authentication and authorization
• add rate limiting and security headers
• improve JSON storage reliability and readiness checks
• document PostgreSQL migration target schema and backend configuration

Validation

CI should run:

• npm run check
• npm test

Review

This is a draft PR. Do not merge yet; review the CI results and changes first.

[coderabbitai]

Note

Currently processing new changes in this PR. This may take a few minutes, please wait...

⚙️ Run configuration

• Configuration used: defaults
• Review profile: CHILL
• Plan: Advanced
• Run ID: 83e9e3c0-8237-42a3-9157-710d5b1360e6

---

📥 Commits

Reviewing files that changed from the base of the PR and between c717a19 and 8909aa6.

---

📒 Files selected for processing (8)

• .github/workflows/ci.yml
• BACKEND.md
• assets/js/admin/queue.js
• auth.html
• docs/postgresql-schema.sql
• package.json
• server.js
• server.test.js

---

 ______________________________________________________________________
< You used `any` like it's a life jacket. Spoiler: it's a pool noodle. >
 ----------------------------------------------------------------------
  \
   \   (\__/)
       (•ㅅ•)
       / 　 づ

✨ Finishing Touches

📝 Generate docstrings

• Commit to this branch
• Create a new PR

---

🧪 Generate unit tests (beta)

• Commit to this branch
• Create a new PR

---

• Autofix · Keep fixing CodeRabbit findings and required CI, and resolving merge conflicts

---

_{Comment @coderabbitai help to get the list of available commands.}