feat(cache): send Age on responses served from a stored entry

[allen0099]

Closes #254

Problem

A cache hit sent Cache-Control: max-age=<ttl> and no Age, so a browser or CDN treated every hit as brand new. A response served just before the backend entry expired could then be reused downstream for another full ttl, up to about twice the ttl in total.

Change

• CacheEntry gains an optional stored_at: float | None = None: epoch seconds from the wall clock (time.time()), because an entry stored by one process or host is served by another. @cache sets it when it stores a response.
• The Redis/Memcached codec writes stored_at. Documents without it (older releases) decode with None. A value that is not a finite number also decodes as None, so a bad field does not turn the entry into a miss. MemoryBackend stores the dataclass as it is, so nothing changes there. Counter entries are unchanged (still a bare integer, no stored_at).
• Responses served from a stored entry send Age: int(clamp(now - stored_at, 0, ttl)) (RFC 9111 §5.1: a non-negative integer). The upper clamp bounds clock skew between hosts: the backend never keeps an entry longer than ttl, so a larger value can only be skew. The lower clamp covers a storing host whose clock runs ahead.
• max-age stays ttl. Downstream computes the remaining freshness as max-age - Age (RFC 9111 §4.2.3). A code comment on _age_headers explains this.
• age joins the headers that are never stored, so a handler's own Age header is not replayed. Replaying it would also have put two Age fields on a hit.
• The time comes from a module-level _now = time.time hook in cache.py, so tests move the clock without sleeping.

Per-path decisions in cache.py

Path	Age?	Why
TTL hit (200 from the stored entry)	yes	Served from storage; this is the case the issue is about.
304 from the stored entry's ETag (If-None-Match, not no_cache)	yes	Answered from the stored entry without running the handler. A cache that refreshes its copy with this 304 takes the headers with it (RFC 9111 §4.3.4), so it needs the entry's real age.
no_cache=True with If-None-Match (304 or 200)	no	The handler re-renders first and the ETag is compared with the fresh response. Nothing comes from storage.
no_cache=True without If-None-Match	no	Always a fresh render.
Miss / fresh render (including a re-store)	no	The response was just produced (age 0 by definition).
no_store, private, ttl of None/0, credential bypass, non-GET	no	The backend is not read. Their 304s compare against a freshly rendered response.
Entry without stored_at (older release)	no	The age is unknown, and a made-up value would be wrong.

Tests

New tests/test_cache_age.py:

• a hit at t+N sends Age: N (fractional N truncated), with max-age unchanged and exactly one Age field;
• an age greater than the ttl is clamped to the ttl, and a negative (skewed) one to 0;
• a 304 from the cached ETag carries Age;
• no Age on a miss, on no_cache revalidation, or on any bypass (no-store, private, ttl=0, non-GET, credential), even with a stored entry under the key;
• no Age on a legacy entry without stored_at (hit and 304);
• a handler's Age is not stored or replayed;
• the codec round-trips stored_at; a legacy document decodes as None; malformed and non-finite values decode as None; counters are unchanged;
• end to end on the memory, Redis and Memcached backends (hit and 304 both carry Age).

Mutation checks (full suite, each mutation reverted afterwards):

• header removed (_age_headers returns {}): 12 failures;
• ttl clamp removed: 2 failures;
• zero clamp removed: 1 failure;
• stored_at not set on store: 12 failures;
• Age dropped from the cached-ETag 304 only: 4 failures;
• codec no longer encodes stored_at: 3 failures (codec round-trip, Redis, Memcached);
• age removed from the unstored headers: 1 failure.

Docs

docs/HTTP_CACHING.md has a new section, "The Age header". docs/CACHE_FLOW.md covers Age in the flow diagram, the decision pseudo-code, the CacheEntry example and the storage formats. The zh-TW translations are updated to match. Changelog fragment: changelog.d/254.fixed.md.