chore: cut 1.2.1 with the docs the sdist will freeze

Dating the changelog and bumping both version strings is the cheap half.
The README is the half that gets frozen into the sdist, so it is fixed
first: the install pin now names 1.2.1 (1.2.0 has no tenant dimension),
and the per-source FIELD table finally lists MISSING_DONE and EXTRA_BODY
- the latter was already referenced elsewhere in the same file. The same
table was missing QUOTA_FULL, the embedding-only keys, the memory cache
backend and three optional PGW_* keys; all are reconciled against
_SOURCE_FIELDS and _load_pgw rather than from memory.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "polygateway"
-version = "1.2.0"
+version = "1.2.1"
 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。