Add a POST /api/admin/v1/room-invites admin endpoint

Mints a single-use, passwordless registration token per recipient, pinned to
their address and optionally to a username, and schedules the invite emails.
The links are only ever mailed to the recipients, so the response carries a
count rather than the tokens.

Minting moves out of the queue and into the caller so a username that is
already taken is refused up front, rather than emailed out as a link nobody
can register with. That leaves the fan-out job with nothing to do, so
`SendRoomInviteEmailsJob` goes away and `SendRoomInviteEmailJob` carries the
token: it takes both the address and the invite link from it, which replaces
the `matrix.to` stub and keeps the link and its recipient together.

`manage send-room-invites` and the endpoint share the minting through
`mas_storage::room_invite`.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/reference/cli/manage.md

@@ -154,8 +154,9 @@ $ mas-cli manage register-user --yes <username>
 
 ## `manage send-room-invites`
 
-Send a room invite email to a list of addresses. This schedules the job; a
-worker must be running to actually send the emails.
+Send a room invite email to a list of addresses. Each address gets a single-use,
+passwordless registration token pinned to it, and a link to register with it.
+This schedules the emails; a worker must be running to actually send them.
 
 Arguments:
 - `<room_id>`: The room to invite the recipients to.