feat(server): impl storage runtime (#15181)

#### PR Dependency Tree


* **PR #15181** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

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

* **New Features**
* Added an additional storage backend option: asset-pack based storage
(provider for avatar, blob, and copilot).
* Introduced a dedicated storage runtime with provider capability
reporting and expanded object operations (put/head/get/list/delete),
including presigned and multipart flows where supported.
* Cloudflare R2 `jurisdiction` now uses an explicit default when
omitted.
* **Bug Fixes**
  * Broadened avatar access to allow both fs and asset-pack providers.
* Improved workspace blob upload completion validation and handling when
stored objects are missing or mismatched.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/native/src/runtime/mod.rs

@@ -0,0 +1,10 @@
+pub mod backend_runtime;
+pub mod storage_runtime;
+
+pub(crate) mod config;
+pub(crate) mod error;
+pub(crate) mod migrations;
+pub(crate) mod types;
+
+pub(crate) use config::BackendRuntimeConfig;
+pub(crate) use error::{RuntimeError, RuntimeResult, napi_error, to_napi_error};