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/README.md

@@ -954,18 +954,11 @@ class PolicyType(StrEnum):
     S3 = "s3"        # S3 兼容存储
 ```
 
-### StorageType
+### PolicyType
 ```python
-class StorageType(StrEnum):
+class PolicyType(StrEnum):
     LOCAL = "local"              # 本地存储
-    QINIU = "qiniu"              # 七牛云
-    TENCENT = "tencent"          # 腾讯云
-    ALIYUN = "aliyun"            # 阿里云
-    ONEDRIVE = "onedrive"        # OneDrive
-    GOOGLE_DRIVE = "google_drive"  # Google Drive
-    DROPBOX = "dropbox"          # Dropbox
-    WEBDAV = "webdav"            # WebDAV
-    REMOTE = "remote"            # 远程存储
+    S3 = "s3"                    # S3 兼容存储
 ```
 
 ### UserStatus