Pushing game-defined realtime events to a player's socket.

Core's own events (updated, notification, member_joined, …) are fixed and documented in GamendWeb.RealtimeEvents. This is the escape hatch a plugin uses for events core knows nothing about — a quest counter, a boss spawn — without needing its own channel:

Gamend.Realtime.push_to_user(user.id, "quest_progress", %{id: 7, step: 2})

Delivery rides the user's existing user:<id> channel, so the client needs no new subscription. The payload is JSON; protobuf mapping is reserved for core events, whose schemas ship with the clients.

The event name must be declared by the plugin's realtime_events/0 callback (see Gamend.Hooks.Declarations), for the same reason notification codes are checked: an undeclared event reaches clients that have no idea it exists, and never appears in the admin runtime page.