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/user.py

@@ -4,7 +4,7 @@ from typing import Literal, TYPE_CHECKING, TypeVar
 from uuid import UUID
 
 from pydantic import BaseModel
-from sqlalchemy import BinaryExpression, ClauseElement, and_
+from sqlalchemy import BigInteger, BinaryExpression, ClauseElement, and_
 from sqlmodel import Field, Relationship
 from sqlmodel.ext.asyncio.session import AsyncSession
 from sqlmodel.main import RelationshipInfo
@@ -473,7 +473,7 @@ class User(UserBase, UUIDTableBaseMixin):
     status: UserStatus = UserStatus.ACTIVE
     """用户状态"""
 
-    storage: int = Field(default=0, sa_column_kwargs={"server_default": "0"}, ge=0)
+    storage: int = Field(default=0, sa_type=BigInteger, sa_column_kwargs={"server_default": "0"}, ge=0)
     """已用存储空间（字节）"""
 
     avatar: str = Field(default="default", max_length=255)