Automatically inform vault owner about "grant permission" requests via email

### Please agree to the following

- [X] I have searched [existing issues](https://github.com/cryptomator/hub/issues?q=) for duplicates
- [X] I agree to follow this project's [Code of Conduct](https://github.com/cryptomator/hub/blob/develop/.github/CODE_OF_CONDUCT.md)

### Summary

I'd like to let vault admins know that I require access to a vault.

### Motivation

Currently, I have to inform the vault admin (and maybe I don't even know who it is) to grant me permission to a vault if I have a new device. It would improve the workflow if the vault admin could be automatically notified via email.

### Considered Alternatives

_No response_

### Anything else?

Hub doesn't know about vault admins yet since it's dependent on the ownership of the vault admin password. We may have to add a "vault admin" role for that.

Start by tracing the existing “grant permission” request flow and how vault ownership is represented, then review how email notifications are handled, if applicable. Clarify whether a vault-admin role is required and define the ownership, notification, and permission-granting behavior before implementation; done means eligible admins are reliably informed of access requests.

Written by the indexing model from the issue text.