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>

main.py

@@ -8,6 +8,7 @@ from routers import router
 from routers.dav import dav_app
 from routers.dav.provider import EventLoopRef
 from service.redis import RedisManager
+from service.storage import S3StorageService
 from sqlmodels.database_connection import DatabaseManager
 from sqlmodels.migration import migration
 from utils import JWT
@@ -50,8 +51,10 @@ lifespan.add_startup(_init_db)
 lifespan.add_startup(migration)
 lifespan.add_startup(JWT.load_secret_key)
 lifespan.add_startup(RedisManager.connect)
+lifespan.add_startup(S3StorageService.initialize_session)
 
 # 添加关闭项
+lifespan.add_shutdown(S3StorageService.close_session)
 lifespan.add_shutdown(DatabaseManager.close)
 lifespan.add_shutdown(RedisManager.disconnect)