Add a broadcast endpoint for notifying multiple users

POST /notifications publishes to exactly one user_id per call. There is no endpoint for notifying a list of users or everyone at once -- anything that wants to announce something to more than one user, such as a maintenance-window notice or a shared-document update, has to loop over POST /notifications itself, once per recipient, from the caller's side. That's N round trips for something that's conceptually one event, and it pushes retry and partial-failure handling onto every caller instead of handling it once, here.

There is currently no route for publishing the same event to more than one user_id in a single call; POST /notifications only accepts a single user_id.

1. pytest practice_tickets/tests/test_ticket_09_broadcast_endpoint.py -v
2. Observe: POST /notifications/broadcast with user_ids=['alice', 'bob'] returns 404 -- there is currently no way to notify more than one user_id in a single POST /notifications call.

This is mostly a matter of looping publish_event() over user_ids and shaping the request/response schema -- the interesting design question is what the response should look like when there's more than one recipient (a dict keyed by user_id, since each publish gets its own independent stream entry id) and what a caller should be able to assume about partial failure.

Look at how app/routers/notifications.py::publish_notification calls publish_event() for a single user_id, and how PublishRequest / PublishResponse are shaped in app/schemas.py. A broadcast is the same operation repeated across a list -- the new request model needs user_ids: list[str] instead of user_id: str, and the response needs to carry one id per recipient instead of one id total.