Webhook Support

[stephenjason89]

Goal

Bring webhook support up to Pusher's webhook spec so backends relying on Pusher webhooks work unchanged.

Current state (as of v1.5.2)

• channel_vacated is implemented (notifyChannelVacancy in src/websocket.ts) with an HMAC X-Pusher-Signature, X-Pusher-Key header, retries with exponential backoff, and a 1s debounce delay.
• Known bug: the debounce is never cancelled when the channel is re-occupied — tracked in Delayed channel_vacated webhook is never cancelled when the channel is re-occupied #10.

Remaining work

• channel_occupied webhook when the first subscriber joins a channel
• member_added / member_removed webhooks for presence channels
• Match Pusher's webhook body format: top-level time_ms alongside events, and per-event fields (name, channel, user_id where applicable) — see Pusher webhooks reference
• client_event webhook once client events land (Support client events (client-*) #13)
• Consider renaming/generalizing subscriptionVacancyUrl to a single webhook URL config since it will carry more than vacancy events
• Robust error handling and structured logging for webhook delivery (original scope of this issue)