Refactor backend APIs to RESTful route patterns.

Align group and conversation handlers/services with path-based endpoints, and unify response/service error handling for related modules.

Made-with: Cursor

internal/service/sticker_service.go

@@ -9,8 +9,8 @@ import (
 )
 
 var (
-	ErrStickerAlreadyExists = errors.New("sticker already exists")
-	ErrInvalidStickerURL    = errors.New("invalid sticker url")
+	ErrStickerAlreadyExists = &ServiceError{Code: 400, Message: "sticker already exists"}
+	ErrInvalidStickerURL    = &ServiceError{Code: 400, Message: "invalid sticker url"}
 )
 
 // StickerService 自定义表情服务接口