Redis cache backend support limits and default configuration in Contao

Contao 4.x documents Redis as an optional cache backend, but the default configuration parameters are not fully specified. This leaves an unresolved decision about expected cache consistency across installations when Redis is enabled.

The goal is to clarify the documented behavior for Redis cache support and identify which settings determine cache hit and miss behavior by default, given that misconfiguration can lead to inconsistent cache behavior.

What configuration values are treated as defaults for the Redis cache backend in Contao? How does Contao select between file-based and Redis caching when both are configured? Which settings are required to achieve consistent cache behavior across installations?