perf(session): write on lookup only when sliding expiration renews

[allen0099]

Closes #115.

What

SessionManager.get_session() saved the session on every lookup just to record last_accessed. Every authenticated request therefore cost a backend write, and a request that also modified the session cost two (the lookup, then the middleware's save).

• get_session() now saves only when sliding expiration renewed the session. That is the only kind of lookup that changes expires_at, and so the backend TTL.
• The new keyword-only touch=True saves on every lookup, for callers who need the stored last_accessed to be exact.
• Session entries store the constant fingerprint "session" instead of a SHA-256 of the payload. Nothing reads session fingerprints: the monitoring routes skip non-HTTP keys, and sessions don't use delete_if_equals.

These are all unchanged:

• expiry checks (an expired session is still saved as EXPIRED and raises);
• absolute_timeout;
• IP / User-Agent binding;
• invalidation;
• regenerate_session_id;
• the middleware's clear()/logout logic, rotation detection and saving of modified data.

Behaviour change

The last_accessed on the returned session is still the current time. The stored last_accessed is now the time of the last write (create, modify, renew, regenerate or touch=True), not the time of the last lookup. Nothing in the library reads it.

A side benefit: a read-only request no longer rewrites the session it loaded at the start. So it can no longer bring back a session that a concurrent request deleted, or overwrite that request's changes with stale data. The same window still exists for requests that modify the session: the middleware saves the copy it loaded. That is out of scope here.

Tests

tests/session/test_lookup_writes.py wraps MemoryBackend in a spy that records every set:

• a plain lookup makes 0 writes, with sliding expiration on and off;
• a renewal makes exactly 1 write and moves the stored expires_at;
• touch=True makes 1 write and stores last_accessed;
• through FastAPICacheXSessionMiddleware, a read-only request makes 0 writes and a modifying request makes exactly 1;
• the deprecated SessionMiddleware lookup makes 0 writes;
• the stored fingerprint is "session".

Mutation check: each change below was applied to manager.py and the full suite was run.

Mutation	Tests that fail
Restore the unconditional save	5 new tests (both plain-lookup cases, touch, both middleware tests)
Drop the save on renewal	The new renewal test, plus the existing test_no_renewal_once_expiry_reaches_absolute_timeout
Ignore touch	Only the touch test
Hash the fingerprint again	Only the fingerprint test

uv run pytest -q: 821 passed, 191 skipped. The live Redis/Memcached tests were skipped and are still to be run. pre-commit run --all-files is clean (the uv-lock hook was skipped because it only bumps unrelated upstream pins; uv lock --check passes). Both strict docs builds (EN and zh-TW) pass.

CHANGELOG

Changed

• Session lookups no longer write to the backend unless sliding expiration renewed the session. SessionManager.get_session() used to save the session on every call to record last_accessed, so each authenticated request cost a write (two if it also modified the session). The stored last_accessed is now updated only when the session is written (created, modified, renewed or regenerated); pass get_session(..., touch=True) to save it on every lookup. Session entries also store a constant fingerprint instead of hashing the payload. (Session lookups write to the backend on every request #115)