fix(redis): warn when the backend is configured with a non-UTF-8 encoding

[allen0099]

Closes #122.

Problem

The shared codec always writes UTF-8 JSON, but the Redis client decodes every reply with the configured encoding. With anything else, non-ASCII content comes back corrupted and nothing reports it. Reproduced on a live Redis:

utf-8    b'\xe9'
latin-1  b'\xc3\xa9'

Change

• AsyncRedisCacheBackend.__init__ emits a RuntimeWarning when encoding is not UTF-8. The name is normalised with codecs.lookup, so "UTF8" and "utf_8" are accepted silently. load_from_config() goes through the same check. The warning points at the caller's line.
• An unknown codec is not warned about: redis-py already raises LookupError when the client is built.
• It warns rather than rejects, so existing configurations keep starting. 0.4.0: drop the Redis encoding option and read raw bytes #126 drops the parameter in 0.4.0.
• The encoding docstring, docs/BACKENDS.md and the changelog explain why the encoding has to stay UTF-8.

Tests

• UTF-8 aliases do not warn. latin-1, utf-16 and ascii warn, attributed to the test file. load_from_config warns. An unknown codec raises LookupError without the warning.
• A live Redis test pins the corruption the warning describes.

With the warning call removed, the five warning tests fail. The full suite passes against live Redis and Memcached (CACHEX_REQUIRE_LIVE_SERVERS=1): 799 passed, 100% coverage. mypy --strict is clean.

This and #156 each add a ### Fixed section to ## [Unreleased]. Whichever merges second needs a small CHANGELOG rebase.