fix(memory): match clear_pattern case-sensitively on every platform

[allen0099]

Closes #179

Summary

• MemoryBackend.clear_pattern() now uses fnmatch.fnmatchcase. fnmatch.fnmatch passes both arguments through os.path.normcase, which lowercases on Windows, so cache:user:* also cleared cache:User:1 there. Redis is case-sensitive everywhere.
• Document in docs/BACKENDS.md (and zh-TW) where the memory glob syntax still differs from Redis.
• CHANGELOG Fixed entry.

Glob differences, checked against a live Redis

Pattern	Memory	Redis
a[*]b	a*b	a*b
a\*b	a\b (backslash is literal)	a*b
a[!x]b	anything but x	a!b, axb
a[^x]b	a^b, axb	anything but x
a?b	same	same

So the docs recommend bracket escapes, and note that negation is [!...] on memory and [^...] on Redis. Making memory accept Redis syntax would change the meaning of existing patterns, so it is left out of this fix.

Tests

• test_memory_backend_clear_pattern_is_case_sensitive_on_windows swaps os.path.normcase for ntpath.normcase. With the fix stashed, it is the only test that fails.
• ruff, mypy --strict, and the full suite against live Redis and Memcached (CACHEX_REQUIRE_LIVE_SERVERS=1): 858 passed.