refactor(cache): remove database fallback from conversation sequence retrieval

Remove the fallback mechanism that queried the database when Redis keys were missing or unavailable. The `GetConvMaxSeq` and `GetConvMaxSeqBatch` methods now strictly rely on the Redis cache and return `ErrKeyNotFound` if the data is not present.

internal/cache/conversation_cache.go

@@ -589,30 +589,23 @@ func (c *ConversationCache) GetUserReadSeqs(ctx context.Context, userID string,
 	return result, nil
 }
 
-// GetConvMaxSeq 获取会话的最大 seq（优先 Redis，降级 DB）
+// GetConvMaxSeq 获取会话的最大 seq（复用 msg_seq 计数器，读取当前值）
 func (c *ConversationCache) GetConvMaxSeq(ctx context.Context, convID string) (int64, error) {
 	seqKey := MessageSeqKey(convID)
 
 	redisCache, ok := c.cache.(*RedisCache)
-	if ok {
+	if !ok {
+		return 0, ErrKeyNotFound
+	}
+
 	rdb := redisCache.GetRedisClient().GetClient()
 	nKey := normalizeKey(seqKey)
 	val, err := rdb.Get(ctx, nKey).Int64()
-		if err == nil {
+	if err != nil {
-			return val, nil
-		}
-	}
-
-	// Redis 未命中或不可用，降级到 DB
-	if c.repo != nil {
-		conv, err := c.repo.GetConversationByID(convID)
-		if err == nil && conv.LastSeq > 0 {
-			return conv.LastSeq, nil
-		}
-	}
-
 		return 0, ErrKeyNotFound
 	}
+	return val, nil
+}
 
 // GetConvMaxSeqBatch 批量获取多个会话的 maxSeq（MGet 减少网络往返）
 func (c *ConversationCache) GetConvMaxSeqBatch(ctx context.Context, convIDs []string) (map[string]int64, error) {
@@ -645,12 +638,6 @@ func (c *ConversationCache) GetConvMaxSeqBatch(ctx context.Context, convIDs []st
 	result := make(map[string]int64, len(convIDs))
 	for i, val := range vals {
 		if val == nil {
-			// Redis 未命中，降级到 DB
-			if c.repo != nil {
-				if conv, err := c.repo.GetConversationByID(convIDs[i]); err == nil && conv.LastSeq > 0 {
-					result[convIDs[i]] = conv.LastSeq
-				}
-			}
 			continue
 		}
 		var seq int64