Function set_object_storage_class 

pub async fn set_object_storage_class(
    client: &Client,
    bucket: &str,
    key: &str,
    new_class: String,
) -> Result<PutResult, AppError>

Change the storage class of bucket/key via a server-side self-copy.

S3 does not expose a dedicated “change storage class” API. The only supported approach is a CopyObject from bucket/key back to itself with StorageClass set to the new value and MetadataDirective::Copy so the existing metadata and tags are preserved.

Errors

• AppError::Validation — new_class is not a recognised S3 storage class string (validation happens at the SDK builder level; unrecognised values pass through to S3 which returns InvalidStorageClass).
• AppError::AccessDenied — s3:CopyObject permission denied.
• AppError::NotFound — bucket or key does not exist.
• AppError::RateLimited — throttling response from AWS.
• AppError::Network — any other SDK or transport error.

Note: not optimistic (Decision D2)

Storage class change is a diff-gated mutation and is intentionally excluded from EXCLUDED_FROM_OPTIMISM in src/query/optimistic.ts. The test storage_class_change_does_not_use_optimistic_path asserts this invariant.