Merge origin/main into nightly

Resolve conflicts:
- .gitignore: keep nightly additions (.test, skills-lock.json)
- relay/helper/price.go: keep both billingexpr and model imports
- en.json / zh-CN.json: keep nightly's superset of i18n entries
- service/billing_session.go: add missing 3rd arg to DecreaseUserQuota
- en.json / zh-CN.json: deduplicate 129+320 duplicate i18n keys

.gitignore

@@ -31,4 +31,4 @@ data/
 .gopath
 .test
 token_estimator_test.go
-skills-lock.json
+skills-lock.json