fix(cache): enhance sequence generation atomicity and concurrency safety

Implement Lua scripts for atomic sequence incrementing and cold-start initialization in Redis to prevent race conditions. Improve the sequence buffer manager with CAS-like logic to prevent interval leakage during concurrent allocations. Update the message repository to use `SELECT ... FOR UPDATE` and conditional updates for `last_seq` to ensure database consistency during high concurrency.

- Add `nextSeqLua` and `initSeqLua` for atomic Redis operations
- Implement thread-safe buffer management in `seq_buffer.go`
- Add row-level locking in `message_repo.go`
- Update service layers to support group-aware sequence retrieval

internal/repository/message_repo.go

@@ -414,14 +414,21 @@ func (r *messageRepository) UpdateConversationLastSeq(conversationID string, seq
 		}).Error
 }
 
-// GetNextSeq 获取会话的下一个seq值
+// GetNextSeq 获取会话的下一个seq值（使用 SELECT ... FOR UPDATE 保证原子性）
 func (r *messageRepository) GetNextSeq(conversationID string) (int64, error) {
-	var conv model.Conversation
-	err := r.db.Select("last_seq").Where("id = ?", conversationID).First(&conv).Error
-	if err != nil {
-		return 0, err
-	}
-	return conv.LastSeq + 1, nil
+	var nextSeq int64
+	err := r.db.Transaction(func(tx *gorm.DB) error {
+		var conv model.Conversation
+		if err := tx.Clauses(clause.Locking{Strength: "UPDATE"}).Where("id = ?", conversationID).First(&conv).Error; err != nil {
+			return err
+		}
+		nextSeq = conv.LastSeq + 1
+		// 立即写回，防止并发读取同一个 last_seq 值
+		return tx.Model(&model.Conversation{}).
+			Where("id = ?", conversationID).
+			Update("last_seq", nextSeq).Error
+	})
+	return nextSeq, err
 }
 
 // CreateMessageWithSeq 创建消息并更新seq（事务操作）
@@ -433,13 +440,20 @@ func (r *messageRepository) CreateMessageWithSeq(msg *model.Message) error {
 			return err
 		}
 
-		// 更新会话的last_seq
-		if err := tx.Model(&model.Conversation{}).
-			Where("id = ?", msg.ConversationID).
+		// 更新会话last_seq：仅当新 seq 更大时才更新，防止并发写入导致 last_seq 回退
+		result := tx.Model(&model.Conversation{}).
+			Where("id = ? AND last_seq < ?", msg.ConversationID, msg.Seq).
 			Updates(map[string]any{
 				"last_seq":      msg.Seq,
 				"last_msg_time": gorm.Expr("CURRENT_TIMESTAMP"),
-			}).Error; err != nil {
+			})
+		if result.Error != nil {
+			return result.Error
+		}
+		// 当 last_seq >= msg.Seq 时（并发写入已更新），仍需刷新 last_msg_time
+		if err := tx.Model(&model.Conversation{}).
+			Where("id = ? AND last_seq >= ?", msg.ConversationID, msg.Seq).
+			Update("last_msg_time", gorm.Expr("CURRENT_TIMESTAMP")).Error; err != nil {
 			return err
 		}