refactor(admin): retire settings migration and compat menu demotion

Stop auto-running AKSRT_Settings_Migration, drop multi-option settings
merge and Zibll favicon aliases, remove the migration tools entry from
menus/schema, flatten admin sections so pages are not demoted into a
compat bag, and retire the migration-status payload for leftover SPA
routes after a full admin rebuild.

assets/admin/src/pages/tools/ToolsImportExport.vue

@@ -8,7 +8,6 @@
         <n-space>
           <n-button quaternary @click="go('/tools/backup')">数据备份</n-button>
           <n-button quaternary @click="go('/tools/health')">健康检查</n-button>
-          <n-button quaternary @click="go('/tools/migration')">迁移状态</n-button>
         </n-space>
       </template>
     </PageHeader>