refactor(admin): clean up audit/permission/paid content config

- P1: remove the duplicate "审核联动" (ai_moderate_background/submit)
  block from the attachment permissions page; those settings live in
  the "评论审核" / API content audit surfaces.
- P2: rename the "内容审核" submenu to "评论审核" (permissions domain).
- P4: remove the "下载/付费内容中心" page and its schema/route chain;
  hardcode runtime policy so download always supports credit purchase
  and paid content is limited to posts. Drop the now-dead
  cms_features.download_credit flag and parse_slug_list helper.

assets/admin/src/pages/permissions/AttachmentPermissions.vue

@@ -70,28 +70,6 @@
         </n-form>
       </n-card>
 
-      <!-- AI Moderation -->
-      <n-card class="aksrt-section-card">
-        <div class="aksrt-section-header">
-          <h3>审核联动</h3>
-        </div>
-        <n-form :model="form" label-placement="left" label-width="200" class="aksrt-form">
-          <n-form-item label="背景图接入 AI 审核">
-            <n-switch v-model:value="form.ai_moderate_background" />
-            <template #feedback>
-              <span class="aksrt-hint-text">命中高风险时会阻断上传，并保留上一次背景图。</span>
-            </template>
-          </n-form-item>
-
-          <n-form-item label="投稿图片接入 AI 审核">
-            <n-switch v-model:value="form.ai_moderate_submit" />
-            <template #feedback>
-              <span class="aksrt-hint-text">投稿中插入的图片也会经过内容安全审核。</span>
-            </template>
-          </n-form-item>
-        </n-form>
-      </n-card>
-
       <div class="aksrt-form-actions">
         <n-button type="primary" :loading="saving" @click="handleSave">保存配置</n-button>
       </div>
@@ -112,15 +90,12 @@ const message = useMessage()
 
 const loading = ref(true)
 const saving = ref(false)
-const moderationSnapshot = ref({})
 
 const form = reactive({
   avatar_max_size: 2,
   submit_image_max_size: 5,
   comment_image_max_size: 2,
   resource_max_size: 50,
-  ai_moderate_background: true,
-  ai_moderate_submit: false,
 })
 
 const summaryItems = computed(() => [
@@ -139,24 +114,15 @@ function goModeration() { router.push('/permissions/moderation') }
 async function load() {
   loading.value = true
   try {
-    const [permissionRes, moderationRes] = await Promise.all([
-      api.getPermissionsAttachmentSettings(),
-      api.getPermissionsModerationSettings(),
-    ])
+    const permissionRes = await api.getPermissionsAttachmentSettings()
 
     const permission = permissionRes?.data ?? permissionRes ?? {}
-    const moderation = moderationRes?.data ?? moderationRes ?? {}
 
     // Map permission fields
     form.avatar_max_size = Number(permission.avatar_max_size || 2)
     form.submit_image_max_size = Number(permission.submit_image_max_size || 5)
     form.comment_image_max_size = Number(permission.comment_image_max_size || 2)
     form.resource_max_size = Number(permission.resource_max_size || 50)
-
-    form.ai_moderate_background = moderation.ai_moderate_background !== false
-    form.ai_moderate_submit = !!moderation.ai_moderate_submit
-
-    moderationSnapshot.value = { ...moderation }
   } catch (error) {
     message.error(error.message || '加载附件权限失败')
   } finally {
@@ -167,24 +133,12 @@ async function load() {
 async function handleSave() {
   saving.value = true
   try {
-    await Promise.all([
-      api.savePermissionsAttachmentSettings({
-        avatar_max_size: form.avatar_max_size,
-        submit_image_max_size: form.submit_image_max_size,
-        comment_image_max_size: form.comment_image_max_size,
-        resource_max_size: form.resource_max_size,
-      }),
-      api.savePermissionsModerationSettings({
-        ...moderationSnapshot.value,
-        ai_moderate_background: form.ai_moderate_background,
-        ai_moderate_submit: form.ai_moderate_submit,
-      }),
-    ])
-    moderationSnapshot.value = {
-      ...moderationSnapshot.value,
-      ai_moderate_background: form.ai_moderate_background,
-      ai_moderate_submit: form.ai_moderate_submit,
-    }
+    await api.savePermissionsAttachmentSettings({
+      avatar_max_size: form.avatar_max_size,
+      submit_image_max_size: form.submit_image_max_size,
+      comment_image_max_size: form.comment_image_max_size,
+      resource_max_size: form.resource_max_size,
+    })
     message.success('附件治理配置已保存')
   } catch (error) {
     message.error(error.message || '保存失败')