Fix Redis setup documentation: correct service name and add environment variable configuration

[Copilot]

Addresses review feedback on PR #2067 to fix Redis installation documentation issues across deployment guides.

Changes

• Ubuntu guide: Corrected systemctl service name from redis to redis-server to match standard Ubuntu package conventions
• Ubuntu guide: Added authentication parameter to redis-cli ping verification for password-protected instances:

  # If you configured a password with requirepass
  redis-cli -a your_secure_password ping
  
  # If no password is set
  redis-cli ping

• Ubuntu guide: Added Redis environment variable configuration section with complete examples for local and managed deployments
• AWS guide: Added Redis environment variable configuration section showing required variables for ElastiCache and serverless providers

All guides now consistently document the required environment variables (REDIS_CACHE, REDIS_HOST, REDIS_PORT, REDIS_PASSWORD, REDIS_TLS) matching the pattern established in the Cloudflare guide.

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