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/cache/keys.go

@@ -38,12 +38,15 @@ const (
 	PrefixChannelsAllList = "channels:all_list"
 
 	// 消息缓存相关
-	keyPrefixMsgHash     = "msg_hash"   // 消息详情 Hash
-	keyPrefixMsgIndex    = "msg_index"  // 消息索引 Sorted Set
-	keyPrefixMsgCount    = "msg_count"  // 消息计数
-	keyPrefixMsgSeq      = "msg_seq"    // Seq 计数器
-	keyPrefixMsgPage     = "msg_page"   // 分页缓存
-	keyPrefixMsgIdempotent = "msg_idem"  // 消息幂等键
+	keyPrefixMsgHash       = "msg_hash"   // 消息详情 Hash
+	keyPrefixMsgIndex      = "msg_index"  // 消息索引 Sorted Set
+	keyPrefixMsgCount      = "msg_count"  // 消息计数
+	keyPrefixMsgSeq        = "msg_seq"    // Seq 计数器
+	keyPrefixMsgPage       = "msg_page"   // 分页缓存
+	keyPrefixMsgIdempotent = "msg_idem"   // 消息幂等键
+
+	// 帖子创建幂等键前缀（clientRequestID -> 已创建的 postID）
+	keyPrefixPostIdempotent = "post_idem"
 )
 
 // PostListKey 生成帖子列表缓存键
@@ -197,6 +200,12 @@ func MessageIdempotentKey(senderID, clientMsgID string) string {
 	return fmt.Sprintf("%s:%s:%s", keyPrefixMsgIdempotent, senderID, clientMsgID)
 }
 
+// PostCreateIdempotentKey 帖子创建幂等键 (userID + clientRequestID -> postID)
+// 用于在 SetNX 原子抢占期间识别同一客户端请求的重试，避免重复发帖。
+func PostCreateIdempotentKey(userID, clientRequestID string) string {
+	return fmt.Sprintf("%s:%s:%s", keyPrefixPostIdempotent, userID, clientRequestID)
+}
+
 // UserReadSeqKey 用户已读位置缓存键（OpenIM 风格 SEQ_USER_READ:{convID}:{userID}）
 func UserReadSeqKey(convID, userID string) string {
 	return fmt.Sprintf("%s:%s:%s", PrefixUserReadSeq, convID, userID)