Plug router for the Pusher REST API:

• POST /apps/{app_id}/events: trigger an event
• POST /apps/{app_id}/batch_events: trigger up to 10 events at once
• GET /apps/{app_id}/channels: list occupied channels (filter_by_prefix, and info=user_count for presence channels)
• GET /apps/{app_id}/channels/{channel}: one channel's info
• GET /: health check; GET /metrics: Prometheus metrics (opt-in)

Signature verification happens on the raw body/query, before any JSON parsing, since the signature covers the exact bytes sent.

The instance name comes in via the plug opts configured in the cowboy dispatch table ({Plug.Cowboy.Handler, {PusherServer.HttpRouter, name: name}}) and is stashed in conn.private by overriding call/2 — the standard Plug technique for parameterizing a Plug.Router.