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/extensions/ExtensionTools.vue

@@ -94,7 +94,6 @@ const healthItems = ref([
 const tools = ref([
   { key: 'import-export', icon: 'fas fa-exchange-alt', bg: '#1890ff', title: '导入导出', desc: '设置导入、导出和重置', route: '/tools/import-export' },
   { key: 'health', icon: 'fas fa-heartbeat', bg: '#52c41a', title: '健康检查', desc: '系统状态检测和诊断', route: '/tools/health' },
-  { key: 'migration', icon: 'fas fa-exchange-alt', bg: '#722ed1', title: '数据迁移', desc: '设置迁移状态和兼容性', route: '/tools/migration' },
   { key: 'email-rollback', icon: 'fas fa-undo', bg: '#fa8c16', title: '邮箱回滚', desc: '邮箱变更 14 天内回滚', route: '/tools/email-rollback' },
 ])