perf(redis): delete each SCAN page as it arrives in clear operations

[allen0099]

Closes #172

Problem

clear(), clear_pattern() and clear_path() scanned the whole matching keyspace into memory before deleting anything. clear_path() also ran an extra EXISTS on the direct key. For clear_path("/p", include_params=True) over 250 keys, the recorded client calls were scan scan scan exists delete delete delete.

Change

• New _delete_matching(pattern) deletes each SCAN page as it arrives and sums the DEL replies. The same call order is now scan delete scan delete scan delete delete. clear(), clear_path() and both passes of clear_pattern() (including the deprecated prefix-stripped retry from 0.4.0: remove the Redis clear_pattern prefix fallback #125) use it.
• clear_path() deletes the direct key with a plain DEL, since DEL already returns 0 for a missing key.
• Deleting keys during a SCAN is safe, because SCAN still returns every key present for the whole iteration. If SCAN repeats a key on a later page (Redis get_all_keys: deduplicate keys returned twice by SCAN #173), that key is already gone, so DEL does not count it twice.
• _scan_keys (used by get_all_keys) and _delete_keys (used by delete_many) are unchanged.

Tests

• test_redis_clear_deletes_each_scan_page_as_it_arrives[clear|clear_pattern|clear_path]: over three pages, the first DEL comes before the last SCAN.
• test_redis_clear_path_does_not_check_exists.
• test_redis_clear_counts_a_key_scan_repeats_once: a repeat is injected on every later page, and clear_pattern still reports the exact total.
• Mutation checks:
  • With redis.py reverted to master, the first four tests fail. The repeat test still passes, because master deduplicated keys before DEL.
  • With deleted += len(page) in place of the DEL reply, only the repeat test fails.
• ruff, mypy --strict, full suite against live Redis and Memcached (CACHEX_REQUIRE_LIVE_SERVERS=1): 866 passed. backends/redis.py coverage is 100%.

CHANGELOG entry

Section: Changed (tracked in #206)

- **Redis clear operations delete page by page.** `clear()`, `clear_pattern()`
  and `clear_path()` delete each SCAN page as it arrives instead of first
  collecting every matching key in memory, and `clear_path()` no longer runs
  an extra `EXISTS` on the direct key.
  ([#172](https://github.com/allen0099/FastAPI-CacheX/issues/172))