refactor(messaging): shift sequence generation responsibility to database transactions

Move the responsibility of sequence (`seq`) allocation from the application/cache layer to the database layer to ensure absolute consistency. Previously, sequences were pre-allocated via Redis/Lua, which introduced complexity in managing synchronization between the cache and the database.

Key changes:
- Implement `CreateMessageWithSeq` in `message_repo.go` using `SELECT FOR UPDATE` to lock the conversation row and atomically increment the `last_seq` within a single transaction.
- Simplify `SeqBufferManager` by removing the complex local buffering and Lua-based pre-allocation logic, reverting to a simpler model.
- Update `chat_service.go`, `group_service.go`, and `message_service.go` to remove manual sequence retrieval, relying instead on the repository's transactional allocation.
- Introduce `SyncConvSeq` in `conversation_cache.go` to perform "write-through" updates to Redis, ensuring the cache remains synchronized with the database's source of truth.
- Improve cold-start handling in `conversation_cache.go` with a new `syncSeqLua` script to prevent stale sequence reads.

internal/service/message_service.go

@@ -2,7 +2,6 @@ package service
 
 import (
 	"context"
-	"fmt"
 	"time"
 
 	"with_you/internal/cache"
@@ -88,21 +87,17 @@ func (s *MessageService) SendMessage(ctx context.Context, senderID, receiverID s
 		Status:         model.MessageStatusNormal,
 	}
 
-	// 从 Redis 获取下一个 seq
-	if s.conversationCache != nil {
-		seq, seqErr := s.conversationCache.GetNextSeqWithGroup(context.Background(), conv.ID, false)
-		if seqErr != nil {
-			return nil, fmt.Errorf("failed to get next seq for conversation %s: %w", conv.ID, seqErr)
-		}
-		msg.Seq = seq
-	}
-
-	// 使用事务创建消息并更新seq
+	// 事务内原子分配 seq + 创建消息（DB 是 seq 唯一来源）
 	err = s.messageRepo.CreateMessageWithSeq(msg)
 	if err != nil {
 		return nil, err
 	}
 
+	// 写透 seq 到 Redis，保持缓存与 DB 一致
+	if s.conversationCache != nil {
+		s.conversationCache.SyncConvSeq(context.Background(), conv.ID, msg.Seq)
+	}
+
 	// 新消息会改变分页结果，先失效分页缓存，避免读到旧列表
 	if s.conversationCache != nil {
 		s.conversationCache.InvalidateMessagePages(conv.ID)