feat: add S3 storage support, policy migration, and quota enforcement

- Add S3StorageService with AWS Signature V4 signing (URI-encoded for non-ASCII keys)
- Add PATCH /object/{id}/policy endpoint for switching storage policies with background migration
- Implement cross-storage file migration service (local <-> S3)
- Replace deprecated StorageType enum with PolicyType (local/s3)
- Implement GET /user/settings/policies endpoint (was 501 stub)
- Add storage quota pre-allocation on upload session creation to prevent concurrent bypass
- Fix BigInteger for max_storage and user.storage to support >2GB values
- Add policy permission validation on upload and directory creation
- Use group's first policy as default on registration instead of hardcoded name
- Define TaskType.POLICY_MIGRATE and extend TaskProps with migration fields

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

service/storage/__init__.py

@@ -3,6 +3,7 @@
 
 提供文件存储相关的服务，包括：
 - 本地存储服务
+- S3 存储服务
 - 命名规则解析器
 - 存储异常定义
 """
@@ -11,6 +12,8 @@ from .exceptions import (
     FileReadError,
     FileWriteError,
     InvalidPathError,
+    S3APIError,
+    S3MultipartUploadError,
     StorageException,
     StorageFileNotFoundError,
     UploadSessionExpiredError,
@@ -25,4 +28,6 @@ from .object import (
     permanently_delete_objects,
     restore_objects,
     soft_delete_objects,
-)
+)
+from .migrate import migrate_file_with_task, migrate_directory_files
+from .s3_storage import S3StorageService