feat(backends)!: make delete() return whether a key was removed

[allen0099]

Closes #71.

BaseCacheBackend.delete() returns whether a key was removed, instead of None.

Changes

• Abstract signature: async def delete(self, key: str) -> bool.
• Memory: True when the key held an entry that had not expired. An expired entry is still dropped but counts as absent, matching _evict (delete_many, clear_pattern, clear_path) and Redis DEL.
• Redis: the DEL count, as a bool.
• Memcached: delete(noreply=False)'s answer. clear_path() now reuses delete() instead of duplicating it.
• Base fallbacks for third-party backends:
  • delete_many() counts only the keys that existed, not the ones attempted.
  • get_and_delete() and delete_if_equals() use the result, so a caller whose delete found the key already gone no longer also wins. They are still non-atomic, as documented.
• Changelog fragment changelog.d/71.changed.md. The migration section MIGRATING_0_4.md#backend-delete (EN and zh-TW) already describes this change and is unchanged.

Tests

• Each backend's own test file asserts True on the first delete and False on the second. Memory also checks that deleting an expired entry returns False.
• A stubbed Memcached test pins noreply=False and the namespaced key, so it runs without a server.
• test_base.py: the delete_many fallback count, and the fallbacks losing a race between their get and delete.
• Ran the full suite against live Redis and Memcached as well as without them.
• Mutation-checked: making each backend return True unconditionally, restoring the old fallbacks, or dropping noreply=False each fails the tests that guard it.

An independent review agent reviewed the change before this PR. It found nothing blocking; its optional suggestions (stubbed Memcached test, fallbacks using the result, clear_path reuse) are included.