Warn when the Redis backend is configured with a non-UTF-8 encoding

[allen0099]

Problem

The shared codec always writes UTF-8 JSON, but the Redis client decodes replies with the configured encoding. With encoding="latin-1", cached bytes are corrupted on the way back (b"\xe9" returns as b"\xc3\xa9").

Proposal

• 0.3.8: warn (or reject) when encoding is not UTF-8, and document it in BACKENDS.md.
• 0.4.0: drop the parameter and read raw bytes (tracked separately).