fix(pkg/cache/disk): LRU 节点类型断言失败时降级而非 panic

items 与 evictList 的不变量一旦被破坏，读、写、删除与淘汰路径上的裸类型断言
会直接崩掉进程。改为带 ok 检查：Get 退化为缓存未命中，Set 报告污染条目，
deleteUnlocked 跳过容量回退，evict 移除坏节点后继续。

新增 cache_corruption_test.go 锁住该行为：去掉守卫后用例会以
「interface conversion: interface {} is string, not *disk.cacheItem」失败，
加上守卫后 4 个用例全通过。

验证：go build 通过；go test ./pkg/cache/disk/ 全绿（含原有 5 个用例）；
golangci-lint 0 issues；check_cordis_architecture.sh 0 violations。

backend/pkg/cache/disk/cache.go

@@ -143,7 +143,10 @@ func (c *Cache) Set(key string, value []byte, ttl time.Duration) error {
 
 	// Update memory tracker
 	if elem, ok := c.items[key]; ok {
-		item := elem.Value.(*cacheItem)
+		item, ok := elem.Value.(*cacheItem)
+		if !ok {
+			return fmt.Errorf("cache: evict list entry for %q has invalid type %T", key, elem.Value)
+		}
 		c.currentSize += size - item.size
 		item.size = size
 		item.expiredAt = expiredAt
@@ -174,7 +177,11 @@ func (c *Cache) Get(key string) ([]byte, error) {
 		return nil, ErrCacheMiss
 	}
 
-	item := elem.Value.(*cacheItem)
+	item, ok := elem.Value.(*cacheItem)
+	if !ok {
+		c.mu.RUnlock()
+		return nil, ErrCacheMiss
+	}
 	if !item.expiredAt.IsZero() && time.Now().After(item.expiredAt) {
 		c.mu.RUnlock()
 		return c.getAndDeleteIfExpired(key)
@@ -224,7 +231,11 @@ func (c *Cache) getAndDeleteIfExpired(key string) ([]byte, error) {
 		return nil, ErrCacheMiss
 	}
 
-	item := elem.Value.(*cacheItem)
+	item, ok := elem.Value.(*cacheItem)
+	if !ok {
+		_ = c.deleteUnlocked(key)
+		return nil, ErrCacheMiss
+	}
 	if !item.expiredAt.IsZero() && time.Now().After(item.expiredAt) {
 		_ = c.deleteUnlocked(key)
 		return nil, ErrCacheMiss
@@ -254,8 +265,9 @@ func (c *Cache) Delete(key string) error {
 
 func (c *Cache) deleteUnlocked(key string) error {
 	if elem, ok := c.items[key]; ok {
-		item := elem.Value.(*cacheItem)
-		c.currentSize -= item.size
+		if item, ok := elem.Value.(*cacheItem); ok {
+			c.currentSize -= item.size
+		}
 		c.evictList.Remove(elem)
 		delete(c.items, key)
 	}
@@ -308,7 +320,11 @@ func (c *Cache) evict() {
 
 	for c.currentSize > c.maxSize && c.evictList.Len() > 0 {
 		elem := c.evictList.Back()
-		item := elem.Value.(*cacheItem)
+		item, ok := elem.Value.(*cacheItem)
+		if !ok {
+			c.evictList.Remove(elem)
+			continue
+		}
 		c.currentSize -= item.size
 		c.evictList.Remove(elem)
 		delete(c.items, item.key)
@@ -400,7 +416,12 @@ func (c *Cache) cleanExpired() {
 
 	now := time.Now()
 	for key, elem := range c.items {
-		item := elem.Value.(*cacheItem)
+		item, ok := elem.Value.(*cacheItem)
+		if !ok {
+			c.evictList.Remove(elem)
+			delete(c.items, key)
+			continue
+		}
 		if !item.expiredAt.IsZero() && now.After(item.expiredAt) {
 			c.currentSize -= item.size
 			c.evictList.Remove(elem)

backend/pkg/cache/disk/cache_corruption_test.go

@@ -0,0 +1,63 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package disk
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+// 这些用例锁住「LRU 链表节点被污染时不得 panic」的行为：一旦 items 与 evictList
+// 的不变量被破坏（例如后续改动误写节点），缓存必须降级为未命中/跳过，
+// 而不是在读、写、删除与淘汰路径上崩掉整个进程。
+
+// corruptEntry 写入一个键后把其链表节点值换成非法类型，返回缓存。
+func corruptEntry(t *testing.T, key string) *Cache {
+	t.Helper()
+
+	c := New(t.TempDir())
+	require.NoError(t, c.Set(key, []byte("payload"), time.Minute))
+
+	elem, ok := c.items[key]
+	require.True(t, ok, "entry must be tracked after Set")
+	elem.Value = "not-a-cacheItem"
+	return c
+}
+
+func TestGetToleratesCorruptEvictEntry(t *testing.T) {
+	c := corruptEntry(t, "k")
+
+	got, err := c.Get("k")
+	require.ErrorIs(t, err, ErrCacheMiss)
+	require.Nil(t, got)
+}
+
+func TestSetOverCorruptEvictEntryReportsError(t *testing.T) {
+	c := corruptEntry(t, "k")
+
+	err := c.Set("k", []byte("second"), time.Minute)
+	require.Error(t, err, "Set must report the corrupted tracker entry instead of panicking")
+	require.Contains(t, err.Error(), "invalid type")
+}
+
+func TestDeleteToleratesCorruptEvictEntry(t *testing.T) {
+	c := corruptEntry(t, "k")
+
+	require.NotPanics(t, func() { _ = c.Delete("k") })
+	require.NotContains(t, c.items, "k")
+}
+
+func TestEvictToleratesCorruptEvictEntry(t *testing.T) {
+	c := corruptEntry(t, "k")
+	// 让任意写入都触发淘汰扫描：扫到被污染的节点必须跳过而非 panic。
+	c.UpdatePolicy(0, 0, true)
+
+	require.NotPanics(t, func() {
+		for i := range 4 {
+			_ = c.Set(string(rune('a'+i)), []byte("x"), time.Minute)
+		}
+	})
+}