2026.7.29-canary.953

feat(server): improve blob sync (#15367)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **New Features**
- Standardized `usePresignedURL` configuration for AWS S3 and Cloudflare
R2 (including `enabled`, `urlPrefix`, and `signKey`).
- Storage upload URL generation now supports both direct provider
presigning and server-mediated proxying based on configuration.
- **Bug Fixes**
- Tightened upload and multipart validation (content type/length checks,
header vs query consistency, and stricter expiration handling).
- Improved fallback behavior when direct upload URL initialization
fails.
- **Tests**
- Updated R2 storage proxy end-to-end coverage to match the new
URL/token behavior.
- **Documentation**
  - Refreshed self-hosted JSON schema guidance for upload URL settings.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->