refactor: replace Trie-based digest session store with flat cache

backend/internal/service/anthropic_session.go

@@ -2,7 +2,6 @@ package service
 
 import (
 	"encoding/json"
-	"strconv"
 	"strings"
 	"time"
 )
@@ -12,9 +11,6 @@ const (
 	// anthropicSessionTTLSeconds Anthropic 会话缓存 TTL（5 分钟）
 	anthropicSessionTTLSeconds = 300
 
-	// anthropicTrieKeyPrefix Anthropic Trie 会话 key 前缀
-	anthropicTrieKeyPrefix = "anthropic:trie:"
-
 	// anthropicDigestSessionKeyPrefix Anthropic 摘要 fallback 会话 key 前缀
 	anthropicDigestSessionKeyPrefix = "anthropic:digest:"
 )
@@ -68,12 +64,6 @@ func rolePrefix(role string) string {
 	}
 }
 
-// BuildAnthropicTrieKey 构建 Anthropic Trie Redis key
-// 格式: anthropic:trie:{groupID}:{prefixHash}
-func BuildAnthropicTrieKey(groupID int64, prefixHash string) string {
-	return anthropicTrieKeyPrefix + strconv.FormatInt(groupID, 10) + ":" + prefixHash
-}
-
 // GenerateAnthropicDigestSessionKey 生成 Anthropic 摘要 fallback 的 sessionKey
 // 组合 prefixHash 前 8 位 + uuid 前 8 位，确保不同会话产生不同的 sessionKey
 func GenerateAnthropicDigestSessionKey(prefixHash, uuid string) string {