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>

sqlmodels/group.py

@@ -2,6 +2,7 @@
 from typing import TYPE_CHECKING
 from uuid import UUID
 
+from sqlalchemy import BigInteger
 from sqlmodel import Field, Relationship, text
 
 from sqlmodel_ext import SQLModelBase, TableBaseMixin, UUIDTableBaseMixin
@@ -260,7 +261,7 @@ class Group(GroupBase, UUIDTableBaseMixin):
     name: str = Field(max_length=255, unique=True)
     """用户组名"""
 
-    max_storage: int = Field(default=0, sa_column_kwargs={"server_default": "0"})
+    max_storage: int = Field(default=0, sa_type=BigInteger, sa_column_kwargs={"server_default": "0"})
     """最大存储空间（字节）"""
 
     share_enabled: bool = Field(default=False, sa_column_kwargs={"server_default": text("false")})