feat(post): add idempotency support for post creation with client request ID

Add client_request_id field to post creation endpoints (CreateVotePost and CreatePostWithSegments).
When provided, the server uses Redis SetNX-based idempotency to prevent duplicate posts
from double-clicks or network retries within a 24-hour TTL. Duplicate requests return
the originally created post; concurrent in-flight requests return 429 "正在发布中，请稍候".

internal/errors/app_errors.go

@@ -130,6 +130,9 @@ var (
 	// 学习资料相关错误
 	ErrSubjectHasMaterials = &AppError{Code: "SUBJECT_HAS_MATERIALS", Message: "学科下存在资料，无法删除"}
 
+	// 帖子创建幂等性：同一 client_request_id 的请求仍在处理中（占位未回填真实结果）
+	ErrDuplicatePostRequest = &AppError{Code: "DUPLICATE_POST_REQUEST", Message: "正在发布中，请稍候"}
+
 	// 身份认证相关错误
 	ErrVerificationPending        = &AppError{Code: "VERIFICATION_PENDING", Message: "已有待审核的认证申请，请等待审核"}
 	ErrVerificationNotFound       = &AppError{Code: "VERIFICATION_NOT_FOUND", Message: "认证记录不存在"}