docs: invalidation guidance, Memcached group invalidation, lock options, stored None, what runs on a hit

[allen0099]

Fixes #329
Fixes #391
Fixes #390
Fixes #389
Fixes #388

Summary

Five documentation gaps, one commit each, English and zh-TW in every commit:

• Docs: dependencies still run on a cache hit; BackgroundTasks run only on a miss #329 HTTP_CACHING.md: new "What still runs on a hit" section. Dependencies run on every request (FastAPI resolves them before the @cache wrapper), a handler's background tasks run only on a miss, and a task added by a dependency's own BackgroundTasks runs on a hit too (checked with a probe app). CACHE_FLOW.md no longer says the handler "does not run at all".
• docs: CacheManager.get() cannot tell a stored None from a miss #391 APP_CACHE.md: get() returns None for a stored None as well as a miss; shows the sentinel-default pattern and points to has() and get_or_set(), which do tell them apart.
• docs: LOCK.md — constructor options and CacheManager.add() for once-per-period jobs #390 LOCK.md: the example shows blocking, timeout and poll_interval on the constructor, and a new "Once per period across workers" section explains why a lock reruns the job and shows CacheManager.add(f"digest:{hour}", True, ttl=3600) instead, including what to do when the job fails.
• docs: group invalidation recipe for CacheManager on Memcached #389 APP_CACHE.md: "Group invalidation on Memcached" recipe: a generation counter read with backend.increment(key, 0) (atomic, creates at 0), embedded in each key, bumped to invalidate; notes the extra read per lookup, the need for a TTL on entries, and bumping by a timestamp step if the backend can lose the counter.
• docs: recommend clear_path() to drop every variant of an updated resource #388 HTTP_CACHING.md: "After a write: invalidate() or clear_path()" explains that invalidate() drops one key while other query strings and Vary variants keep the old data, when to use clear_path(path, include_params=True), and that Memcached needs the docs: group invalidation recipe for CacheManager on Memcached #389 recipe instead.

Both sites build with zensical build --strict. Docs only, no changelog fragment.

Checklist

• The issues above are assigned to me
• Tests under tests/ cover the change (not applicable: docs)
• A changelog fragment changelog.d/<issue>.<section>.md (required for any change under fastapi_cachex/) (not applicable)
• uv run pre-commit run --all-files and uv run pytest pass

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