chore: cut 1.2.3 and date its changelog entry

Dates the unreleased section as 1.2.3 (2026-08-19) and moves both version
strings from 1.2.1 in lockstep. The human picked a patch number knowing
this release carries five breaking changes; that is deliberate.

Two lines added to the upgrade hints: the install pin move, matching what
1.2.1 recorded for its own, and a pointer saying the zero-row RLS
self-check now also lives in the README, since CHANGELOG.md never reaches
anyone who only reads the packaged README.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "polygateway"
-version = "1.2.1"
+version = "1.2.3"
 description = "PolyGateway：实验室统一的大语言模型（LLM/VLM/OCR）调度与中转库——多源、限流、重试、熔断、缓存、遥测"
 # registry 包页面的正文只认这一项:缺了页面就是一片空白(1.1.2 的教训,twine 会警告
 # long_description missing 但不阻塞上传)。README 在打包时被固化进产物,发布后再改无效。