Content-addressed store backends for sui.

This crate owns the [StorageBackend] trait and every concrete implementation (Local / S3 / Redis / Postgres / Tiered), plus the [BackendConfig] config-select enum and the [build_backend] factory. Both sui-cache (the Nix binary-cache server) and sui-registry (the OCI registry porto) depend on this crate; neither depends on the other.

Feature flags

• redis-client — enables the production [RedisConnectionManager] transport. Without it the Redis arm of [build_backend] returns a typed [StoreError::NotImplemented].
• postgres — enables the production [SqlxPgCacheConn] transport. Without it the Postgres arm returns [StoreError::NotImplemented].