feat(backup): 实现自动备份功能及错误边界处理

backend/config/auto-backup-settings.json

@@ -0,0 +1,10 @@
+{
+  "enabled": true,
+  "cronExpression": "0 20 * * *",
+  "description": "自动备份",
+  "backupType": "full",
+  "includeFiles": true,
+  "compress": true,
+  "maxCount": 30,
+  "maxAgeDays": 90
+}

backend/config/constants.js

@@ -11,7 +11,7 @@ module.exports = {
   },
   
   FILE_UPLOAD: {
-    MAX_FILE_SIZE: (parseInt(process.env.MAX_FILE_SIZE_MB, 10) || 50) * 1024 * 1024,
+    MAX_FILE_SIZE: (parseInt(process.env.MAX_FILE_SIZE_MB, 10) || 500) * 1024 * 1024,
     MAX_AVATAR_SIZE: (parseInt(process.env.MAX_AVATAR_SIZE_MB, 10) || 5) * 1024 * 1024,
     ALLOWED_IMAGE_TYPES: ['image/jpeg', 'image/png', 'image/gif', 'image/webp'],
     ALLOWED_DOC_TYPES: [