feat: v2.0 Sprint 2 - Cloud Storage Support (Part 1)

- Add AWS SDK v2 for S3 integration
- Implement cloud.Backend interface for multi-provider support
- Add full S3 backend with upload/download/list/delete
- Support MinIO and Backblaze B2 (S3-compatible)
- Implement progress tracking for uploads/downloads
- Add cloud commands: upload, download, list, delete

New commands:
- dbbackup cloud upload [files] - Upload backups to cloud
- dbbackup cloud download [remote] [local] - Download from cloud
- dbbackup cloud list [prefix] - List cloud backups
- dbbackup cloud delete [remote] - Delete from cloud

Configuration via flags or environment:
- --cloud-provider, --cloud-bucket, --cloud-region
- --cloud-endpoint (for MinIO/B2)
- --cloud-access-key, --cloud-secret-key

New packages:
- internal/cloud - Cloud storage abstraction layer

go.sum

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