i18n: Translate all Chinese to English across the codebase

Translate Chinese comments, log/error messages, validator labels and
Swagger annotations to English throughout the source code, generated
Swagger docs, config files and CI workflows.

Make the English README primary: README.md now holds the English docs
and README_EN.md holds the Chinese version, with cross-language links
updated accordingly.

Note: the generated docs/ swagger files were translated in place; run
`go generate ./...` (swag init) to regenerate them from the now-English
annotations when a Go toolchain is available.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

model/group.go

@@ -1,8 +1,8 @@
 package model
 
 const (
-	GroupTypeDefault = 1 // 默认
-	GroupTypeShare   = 2 // 共享
+	GroupTypeDefault = 1 // default
+	GroupTypeShare   = 2 // shared
 )
 
 type Group struct {