Allow changing repository password from the UI

[freefelt]

Problem

Currently, the repository password field is disabled when editing an existing repository — but the current value is still visible and copyable via the eye icon next to the field. So the lock doesn't protect against accidental exposure; it just blocks changing a value that's already fully visible anyway.

Current workaround

1. Run restic key passwd manually to re-encrypt the master key with a new password: restic -r <repo-uri> key passwd
2. Manually update the stored password in Backrest so it can keep authenticating on scheduled runs — either by editing config.json directly, or by unlocking the disabled field via browser devtools.

Both work, but this is fragile (config.json editing has no validation) and not discoverable for less technical users.

Proposed feature

A "Change Password" action on the repository page that:

1. Prompts for the new password
2. Internally runs restic key passwd (or key add + key remove for the old key) using the repo's stored connection details
3. Updates the stored password in the config automatically on success

Since Backrest already shells out to restic for other operations (snapshots, diff, prune, check), this seems like a small addition on top of existing plumbing — and it would remove any need for manual config.json editing.