feat(cache): implement database fallback for conversation sequence management

Improve the reliability of conversation sequence generation by implementing a fallback mechanism to the database when Redis is unavailable or when keys are missing.

- Add `GetNextSeq` to `MessageRepository` and implement it in `MessageRepositoryAdapter`.
- Update `ConversationCache` to fallback to DB in `GetConvMaxSeq`, `GetConvMaxSeqBatch`, and `GetNextSeq`.
- Ensure Redis TTL is refreshed after every `INCR` operation to prevent sequence counter expiration.
- Refactor service layers to handle sequence generation errors more explicitly.
- Add `IsEnabled` helper to `LayeredCache` for cleaner availability checks.

internal/service/message_service.go

@@ -2,6 +2,7 @@ package service
 
 import (
 	"context"
+	"fmt"
 	"time"
 
 	"with_you/internal/cache"
@@ -90,13 +91,9 @@ func (s *MessageService) SendMessage(ctx context.Context, senderID, receiverID s
 	if s.conversationCache != nil {
 		seq, seqErr := s.conversationCache.GetNextSeq(context.Background(), conv.ID)
 		if seqErr != nil {
-			zap.L().Warn("redis get next seq failed, falling back to DB",
-				zap.String("convID", conv.ID),
-				zap.Error(seqErr),
-			)
-		} else {
-			msg.Seq = seq
+			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