fix(system-settings): 修复线上主题/排期/聚合登录等配置缺失，对新 DB 字段用 SYSTEM_SETTINGS_DEFAULTS 兜底 + 更新接口允许写入新字段 + 缩短 Redis 缓存 TTL

- site-config.get.ts：filterPublicSettings 和 Redis/DB 命中分支全部先合并 SYSTEM_SETTINGS_DEFAULTS，解决老库无 schema 新字段导致 public 返回 defaultTheme=light、enabledThemes=light,dark 等旧值的问题
- admin/system-settings/index.post.ts：新增 ~40 个白名单字段（主题/排期可见范围/留言自动审查/OAuth 聚合/首次强制改密/投稿限制/卡密/注册审核/自动备份/统计代码等），支持管理面板保存生效；更新返回结果用 fillDefaults(DEFAULTS) 兜底
- admin/system-settings/index.ts (GET)：返回 DB 记录前合并 SYSTEM_SETTINGS_DEFAULTS，避免管理后台 UI 拿不到新字段
- cacheService.ts：CACHE_TTL.SYSTEM_SETTINGS 3600s → 900s（60min→15min），防止 Redis 单机挂或旧脏值导致配置变更长时间不生效

server/api/site-config.get.ts

@@ -2,6 +2,7 @@ import { db } from '~/drizzle/db'
 import { systemSettings } from '~/drizzle/schema'
 import { cacheService } from '../services/cacheService'
 import { isRedisReady } from '../utils/redis'
+import { SYSTEM_SETTINGS_DEFAULTS } from '~~/server/utils/system-settings-defaults'
 
 // 过滤敏感字段，只返回公开配置
 const publicFields = [
@@ -36,13 +37,11 @@ const publicFields = [
 ]
 
 const filterPublicSettings = (data: any) => {
-  if (!data) {
-    return {}
-  }
+  const merged: any = { ...SYSTEM_SETTINGS_DEFAULTS, ...(data || {}) }
   const result: Record<string, any> = {}
   for (const key of publicFields) {
-    if (Object.prototype.hasOwnProperty.call(data, key)) {
-      result[key] = data[key]
+    if (Object.prototype.hasOwnProperty.call(merged, key)) {
+      result[key] = merged[key]
     }
   }
   return result
@@ -55,7 +54,7 @@ export default defineEventHandler(async (event) => {
       const cachedSettings = await cacheService.getSystemSettings()
       if (cachedSettings) {
         console.log('[API] 系统设置缓存命中')
-        return filterPublicSettings(cachedSettings)
+        return filterPublicSettings({ ...SYSTEM_SETTINGS_DEFAULTS, ...cachedSettings })
       }
     }
 
@@ -100,7 +99,7 @@ export default defineEventHandler(async (event) => {
       settings = newSettings[0]
     }
 
-    const publicSettings = filterPublicSettings(settings)
+    const publicSettings = filterPublicSettings({ ...SYSTEM_SETTINGS_DEFAULTS, ...(settings || {}) })
 
     // 将结果缓存到Redis（如果可用）- 永久缓存
     if (settings && isRedisReady()) {