fix(cache): skip the backend for routes without a positive ttl

[allen0099]

Closes #110.

Problem

With @cache(ttl=None) (and ttl=0), the response was stored without expiry. It was never served directly, but a request whose If-None-Match matched the stored ETag got a 304 without the handler running. Once the data changed, a client revalidating with the old ETag kept getting 304 until some other request happened to rewrite the entry. The entries also never expired, so every query-string variant accumulated in the backend.

Decision

ttl is what is sent as max-age: how long a stored response may be reused without running the handler. Without a positive ttl, no reuse is allowed. A 304 answered from a stored ETag is a form of reuse: it confirms a copy the handler may no longer produce. So a route without a positive ttl now has no server-side cache at all.

ETag revalidation itself stays. The handler runs and its fresh ETag is compared with If-None-Match, which is always correct and still saves the body transfer. This is option A from the issue (compare against a fresh render) combined with option B (do not store).

Change

• cache.py: bypass_backend = private or not ttl. Routes without a positive ttl take the existing private=True path, which never touches the backend. The now-redundant store_ttl is gone.
• Entries that 0.3.7 stored for such routes are neither read nor refreshed. The changelog mentions that clear() removes them.
• The ttl and no_cache docstrings, docs/HTTP_CACHING.md, docs/CACHE_FLOW.md (including the decision-logic sketch and the scenario table) and the changelog are updated.

No API change. The one behaviour change besides the fix: a request with a matching If-None-Match now runs the handler on these routes.

Tests

• test_without_a_positive_ttl_nothing_is_stored_or_served, run for ttl=None and ttl=0. It replaces the old ttl=0 test, which asserted the stored-ETag 304.
  • Nothing is stored.
  • An unchanged response still gets 304.
  • After the data changes, the old ETag gets 200 with the new body.
• test_without_a_ttl_an_entry_left_by_an_older_version_is_ignored: a pre-existing entry is neither served nor overwritten.
• no_cache tests that exercised the stored path through ttl-less routes now use ttl=60, so the no_cache branches stay covered. test_error_does_not_overwrite_a_good_cached_entry also uses ttl=60, no_cache=True, since it needs the handler to run while an entry is stored.

With bypass_backend = private restored, the three new tests fail. The full suite passes against live Redis and Memcached (CACHEX_REQUIRE_LIVE_SERVERS=1): 792 passed, 100% coverage. mypy --strict and pre-commit are clean.

Like #156, #157 and #158, this adds a ### Fixed section to ## [Unreleased], so later merges need a small CHANGELOG rebase.