store: Link restored deployment to subgraph name

Replace the `assign_subgraph` call in `restore()` with
`create_subgraph_version`, which creates the `subgraph_version` record,
updates `subgraph.current_version`, and creates the assignment. This
ensures the restored deployment is properly connected to its subgraph
name rather than being left orphaned.

Move `restore()` from `impl Inner` to `impl SubgraphStore` so it can
use `self.cheap_clone()` for the `exists_and_synced` closure, matching
the pattern used in `create_deployment_internal`.