feat: add Anthropic sticky session digest chain matching via Trie

The previous fallback (step 3) in GenerateSessionHash hashed system +
all messages together, producing a different hash each round as the
conversation grew ([a] -> [a,b] -> [a,b,c]). This made fallback sticky
sessions ineffective for multi-turn conversations.

Implement per-message Trie digest chain matching (reusing Gemini's Trie
infrastructure) so that the previous round's chain is always a prefix
of the current round's chain, enabling reliable session affinity.

backend/internal/service/anthropic_session.go

@@ -0,0 +1,89 @@
+package service
+
+import (
+	"encoding/json"
+	"strconv"
+	"strings"
+	"time"
+)
+
+// Anthropic 会话 Fallback 相关常量
+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:"
+)
+
+// AnthropicSessionTTL 返回 Anthropic 会话缓存 TTL
+func AnthropicSessionTTL() time.Duration {
+	return anthropicSessionTTLSeconds * time.Second
+}
+
+// BuildAnthropicDigestChain 根据 Anthropic 请求生成摘要链
+// 格式: s:<hash>-u:<hash>-a:<hash>-u:<hash>-...
+// s = system, u = user, a = assistant
+func BuildAnthropicDigestChain(parsed *ParsedRequest) string {
+	if parsed == nil {
+		return ""
+	}
+
+	var parts []string
+
+	// 1. system prompt
+	if parsed.System != nil {
+		systemData, _ := json.Marshal(parsed.System)
+		if len(systemData) > 0 && string(systemData) != "null" {
+			parts = append(parts, "s:"+shortHash(systemData))
+		}
+	}
+
+	// 2. messages
+	for _, msg := range parsed.Messages {
+		msgMap, ok := msg.(map[string]any)
+		if !ok {
+			continue
+		}
+		role, _ := msgMap["role"].(string)
+		prefix := rolePrefix(role)
+		content := msgMap["content"]
+		contentData, _ := json.Marshal(content)
+		parts = append(parts, prefix+":"+shortHash(contentData))
+	}
+
+	return strings.Join(parts, "-")
+}
+
+// rolePrefix 将 Anthropic 的 role 映射为单字符前缀
+func rolePrefix(role string) string {
+	switch role {
+	case "assistant":
+		return "a"
+	default:
+		return "u"
+	}
+}
+
+// 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 {
+	prefix := prefixHash
+	if len(prefixHash) >= 8 {
+		prefix = prefixHash[:8]
+	}
+	uuidPart := uuid
+	if len(uuid) >= 8 {
+		uuidPart = uuid[:8]
+	}
+	return anthropicDigestSessionKeyPrefix + prefix + ":" + uuidPart
+}