docs(changelog): add the 0.3.8 entries collected from parallel PRs

[allen0099]

Summary

Closes #206

Copies the "CHANGELOG" sections from the 0.3.8 PR descriptions into ## [Unreleased], appended to the matching sections. No version heading, date or version change; that belongs to the release.

I checked every PR merged into master since v0.3.7, not only the ones listed in #206.

PR	Issue	Entry	Status
#207	#160	Added: exceptions exported from fastapi_cachex	added here
#208	#165	Fixed: clear_expired_sessions() removes non-active sessions	added here
#209	#168	Fixed: session middleware varies on the token header	added here
#210	#183	None (internal refactor)	no entry
#211	#172	Changed: Redis clear operations delete page by page	added here
#212	#171	Changed: Redis get_cache_data() no longer blocks	added here
#272	#231, #232	Security: releases from master only; Fixed: LICENSE shipped	added here
#273	#174, #176	Changed: Memcached one worker call per operation	added here, reworded (below)
#274	#112	Changed: CacheBackend falls back to MemoryBackend	added here
#275	#114, #116	Added: require_user_session; Security: short JWT key warning	added here
#276	#184, #182	Added: media_type in /cached-records; Changed: key built only when used	added here
#277	#190–#192	None (tooling)	no entry
#278	#186	None (test-only)	no entry
#279	#115	Changed: session lookups no longer write	added here, wrapped and linked
#281	#187	None (test-only)	no entry
#282	#193	None (CI only)	no entry
#283	#188	None (test-only)	no entry
#284	#194	Changed: runtime dependency floors	added here
#263	#230	Security: | and % escaped in cache keys	already in the file

The other PRs merged since v0.3.7 either edited CHANGELOG.md themselves (#150, #153, #156–#159, #195, #196, #198–#200, #203–#205, #215–#218, #220, #221, #223, #224, #255, #257–#259, #262) or changed nothing users see (#149, #151, #152, #154, #155, #219).

Still open and expected to need no entry: #185 and #189 (both test-only).

Wording changes

• perf(memcached): one worker call per operation; delete_many counts removals #273: its entry listed expire_if_equals among the methods that "used to" take several worker-thread trips, and said it now rejects a ttl past 2038 before I/O. expire_if_equals is new in 0.3.8 (feat: add CacheLock and the expire_if_equals backend primitive #150), so neither is a change for users; both were dropped. The rest is verbatim.
• refactor(cache): build the key only where the backend is used; report media_type #276 and perf(session): write on lookup only when sliding expiration renews #279: the text is unchanged, only re-wrapped at ~78 columns. perf(session): write on lookup only when sliding expiration renews #279's bare (#115) became a link like the other entries.
• feat(proxy): one locked get_or_create for lazy defaults; CacheBackend falls back #274 and build(deps): declare lower bounds and test them with a lowest-direct resolution #284: verbatim.

Verification

• uv run pytest tests/test_changelog_release.py: 25 passed.
• scripts/changelog_release.py --version 0.3.8 --dry-run renders all 46 summaries in the five sections. It printed only; nothing was written.
• The strict docs builds passed in EN (which includes the changelog) and zh-TW.
• pre-commit passed.

CHANGELOG

None.

Update

• Rebased onto master after docs: pre-release docs pass for 0.3.8 #290.
• Added a ### Documentation section (as in 0.3.7) with two entries: runnable examples (docs: add runnable examples #291, part of Test the built wheel and the documented examples #241) and the pre-release docs pass (docs: pre-release docs pass for 0.3.8 #290, Bring all docs up to date before releasing 0.3.8 #214). docs: add runnable examples #291's entry is taken verbatim from its PR body, plus the issue link.

Update 2

Added the ### Documentation entry for #294 (session login docs). Merge #294 before or together with this PR.