ci(backend): format check, lint, and test pipeline

[codebestia]

Summary

• Added .prettierrc.json at repo root (single-quote, semi, 100 print width, trailing commas)
• Added eslint.config.js in apps/backend using @eslint/js + typescript-eslint recommended rules
• Added vitest.config.ts with a global setupFiles entry that sets JWT_SECRET before any module is imported
• Wrote 11 unit tests across two suites:
  • jwt.test.ts — sign, verify, tampered token, expired token
  • nonce.test.ts — create, consume, single-use, wrong nonce, unknown wallet, expiry (fake timers)
• Fixed formatting in socketAuth.ts, conversations.ts, messaging.ts to pass Prettier check
• Added scripts to package.json: test, test:watch, test:coverage, lint, format:check
• .github/workflows/backend-ci.yml — triggers on push/PR to apps/backend/**, runs three sequential steps:
  1. Format check — pnpm format:check
  2. Lint — pnpm lint
  3. Tests — pnpm test (uses JWT_SECRET repo secret with a CI fallback)