fix: 上传文件大小限制从 800MB 提升至 2000MB

P0 修复:
- config.py: OSS_DIRECT_UPLOAD_MAX_MB 默认值从 800 改为 2000
- config.py: 添加 AliasChoices 支持 MAX_UPLOAD_SIZE_MB 环境变量
  （.env.production 中 MAX_UPLOAD_SIZE_MB=2000 现在可被正确读取）

P1 修复:
- infra/nginx/xiaoxia-saas.conf: client_max_body_size 800m → 2g（3处）
- infra/docker/nginx-production.conf: client_max_body_size 800m → 2g

apps/api/app/config.py

@@ -1,7 +1,7 @@
 import os
 from typing import Optional
 
-from pydantic import field_validator
+from pydantic import AliasChoices, Field, field_validator
 from pydantic_settings import BaseSettings, SettingsConfigDict
 
 
@@ -83,7 +83,10 @@ class Settings(BaseSettings):
     OSS_ACCESS_KEY_ID: str = ""
     OSS_ACCESS_KEY_SECRET: str = ""
     OSS_BUCKET_NAME: str = "xiaoxia-autocut"
-    OSS_DIRECT_UPLOAD_MAX_MB: int = 800
+    OSS_DIRECT_UPLOAD_MAX_MB: int = Field(
+        default=2000,
+        validation_alias=AliasChoices("OSS_DIRECT_UPLOAD_MAX_MB", "MAX_UPLOAD_SIZE_MB"),
+    )
     OSS_DIRECT_UPLOAD_EXPRESS_SECRET: int = 900
 
     LOG_LEVEL: str = "INFO"