feat(security): implement secure JWT secrets for all environments

- Replace short dev secrets with secure 64-character (256-bit) secrets
- Update docker-compose.yml with secure development secret
- Update docker-compose.local.yml with unique secure secret
- Improve .env.example with clear security instructions
- Add comprehensive security best practices documentation
- Create helper script to generate secure JWT secrets

Security improvements:
- All environments now use cryptographically secure 64-character secrets
- Clear warnings and instructions prevent use of weak secrets
- Documentation guides proper secret management
- Helper script makes it easy to generate new secrets

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

docker-compose.local.yml

@@ -10,7 +10,7 @@ services:
     environment:
       - NODE_ENV=development
       - PORT=3000
-      - JWT_SECRET=local-dev-secret-key-123
+      - JWT_SECRET=b375996f704bb1541ad9297e7710a215fce0b59ecf1c43236aeea32ec01e7b27
       - ADMIN_URL=http://localhost:3005
       - FRONTEND_URL=http://localhost:3005
       # Email - uses Mailhog