fix(backend): avoid duplicate REDIS_URL in env example

[pengyuayn]

Summary

• Keeps the Docker default REDIS_URL=redis://redis:6379 as the single live Redis URL in backend/.env.example.
• Moves the local Redis alternative to a commented example so copied env files do not override the Docker service hostname.
• Verified no other keys are duplicated in backend/.env.example.

Validation

• PASS: duplicate-key scan over backend/.env.example
• PASS: git diff --check

Closes #1209