feat(cache): allow REDIS_KEY_PREFIX env var to override cache KEY_PREFIX

[mulldug]

ref: https://app.clickup.com/t/9014802374/86bbqgc34

Summary

• Replaces the hardcoded `KEY_PREFIX` in `CACHES` with `os.getenv("REDIS_KEY_PREFIX", "")`
• Default behaviour is unchanged — existing deployments without the env var set continue to use the same prefix as before
• Enables operators to override the prefix per-service when multiple services share a single Valkey/Redis DB

Deployment steps

No action required for existing deployments. To use a shared Valkey DB, set `REDIS_KEY_PREFIX=` in the service's environment before deploying.

[coderabbitai]

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