fix(session): clear inactive sessions and batch session sweeps

[allen0099]

Closes #165

Problem

• clear_expired_sessions() only checked expires_at. A record already marked INVALIDATED (by invalidate_session()) or EXPIRED (by an expired read) is unusable, yet it stayed in the backend until its TTL ran out. Repro: create a session, invalidate it, then call clear_expired_sessions(). It returns 0 and the key remains.
• clear_expired_sessions() and delete_user_sessions() sent one sequential delete per session.

Fix

• clear_expired_sessions() now removes any session whose status is not ACTIVE, as well as any session past expires_at.
• Both sweeps collect the matching keys and remove them with a single backend.delete_many() call. The returned count comes from delete_many. On memory and Redis, that is what was actually removed. On the base fallback, it is what was attempted.
• docs/SESSION.md and zh-TW describe what clear_expired_sessions() removes and the single batch delete.

Tests

• test_clear_expired_sessions_removes_sessions_no_longer_active[invalidated|expired]
• test_session_sweeps_delete_in_one_batch: each sweep makes exactly one delete_many call with the right number of keys, and no per-key delete calls.
• Mutation check: with the manager.py change reverted, exactly these 3 tests fail (208 others pass).
• ruff, mypy --strict, and the full suite against live Redis and Memcached (CACHEX_REQUIRE_LIVE_SERVERS=1): 864 passed.

CHANGELOG entry

Section: Fixed (added to CHANGELOG.md via #206)

- **`clear_expired_sessions()` also removes invalidated and expired-status
  sessions.** It only checked `expires_at`, so a session marked `INVALIDATED`
  by `invalidate_session()` or `EXPIRED` by an expired read stayed in the
  backend until its TTL ran out. Any session that is no longer `ACTIVE` is now
  removed. `clear_expired_sessions()` and `delete_user_sessions()` also delete
  what they find with one `backend.delete_many()` call instead of one `delete`
  per session.
  ([#165](https://github.com/allen0099/FastAPI-CacheX/issues/165))