Add REDIS_PASSWORD environment variable support to both Phoenix.PubSub.Redis
and Exq background job processor. Password is optional - if not set, connects
without authentication.
Changes:
- application.ex: Add password to PubSub Redis config if REDIS_PASSWORD is set
- exq_supervisor.ex: Add password to Exq Redis config if REDIS_PASSWORD is set
Required for connecting to Proxmox-hosted Redis with authentication.