fix(redis): match the key prefix and clear_path path literally in SCAN

[allen0099]

Closes #106

Problem

AsyncRedisCacheBackend built its SCAN MATCH patterns by interpolating key_prefix and the clear_path() path directly, so glob metacharacters in them were live:

• clear_path("/files/[draft]") read [draft] as a character class and missed the cached HTTP entry for that path. The memory backend cleared it.
• A key_prefix containing ? or * let clear(), get_all_keys() (and so get_cache_data()) and clear_pattern() reach keys under other prefixes on the same server.

Change

• New _escape_glob() backslash-escapes *?[]\ for Redis glob patterns.
• The key prefix is escaped in clear, get_all_keys, clear_path and clear_pattern.
• The path is escaped in clear_path.
• clear_pattern(pattern) now always builds the pattern as escaped prefix + pattern. A pattern that already repeats the prefix has it stripped first, so both call forms keep working. Only the caller's pattern stays a live glob.
• The Redis section of docs/BACKENDS.md gains a note on this, and there is a CHANGELOG entry under Fixed.

Not breaking.

Out of scope

CacheManager.clear_pattern() still puts its own key_prefix (cache: by default) into the pattern it hands to the backend, so glob characters in a custom manager prefix remain live. The escaping syntax differs per backend (Redis uses backslashes, the memory backend's fnmatch uses [*]), so fixing that needs its own design.

Tests

The new live Redis tests in tests/backends/test_redis.py fail on the old code and pass with this change:

• clear_path on a path containing []*?\ removes its exact and query-param entries and leaves look-alike keys that the unescaped pattern would have matched.
• A backend with prefix cachex-test?*: does not list or delete keys of a cachex-testX-other: backend through get_all_keys, clear_pattern (with and without the prefix) or clear.

Full suite with live Redis and Memcached: 761 passed. The docs build passes with --strict.