feat: added configurable redis cache for shared redis instances

[SahilJat]

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Changes

Closes #7297

Adds a CACHE_KEY_PREFIX environment variable that sets KEY_PREFIX on all Redis-backed cache entries in the CACHES dict.

• Added CACHE_KEY_PREFIX = env.str("CACHE_KEY_PREFIX", default="") to api/app/settings/common.py
• Added a loop after all CACHES entries are defined (including SAML) that applies KEY_PREFIX = CACHE_KEY_PREFIX to any cache whose BACKEND contains "redis" (case-insensitive)
• Non-Redis backends (locmem, db, dummy) are unaffected
• Default is empty string which preserves existing Django behaviour — no breaking change for existing deployments

With CACHE_KEY_PREFIX=flagsmith, Redis keys become flagsmith:1:environment_document_xxx instead of :1:environment_document_xxx, enabling Redis ACL rules like ~flagsmith:* for namespace-based multi-tenancy.

How did you test this code?

• Added two unit tests in api/tests/unit/app/test_unit_app_settings.py:
  • Verifies Redis-backed caches get KEY_PREFIX applied
  • Verifies non-Redis caches (locmem, db) are not affected
  • Verifies empty prefix (default) sets KEY_PREFIX to empty string
• Verified settings load correctly with CACHE_KEY_PREFIX=flagsmith python -c "from app.settings.common import CACHES"
• All existing tests in api/tests/unit/app/ pass

[claude]

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[SahilJat]

Hii alll the test cases has been passed , the test that failed is already exisiting issue mentioned here #7368