Add programmatic webhook registration API

Automation platforms (Activepieces, Make, n8n, Zapier) need to register and clean up webhooks automatically when a user activates or disables a trigger. This only works if the target app exposes two standard endpoints:

• POST /v1/webhooks — register a URL with a list of events

• DELETE /v1/webhooks/{id} — remove it when the flow is disabled

Without these, every user must manually copy a webhook URL from their automation platform and paste it into the Senja dashboard. This is a friction point for non-technical users, and it means orphaned webhooks accumulate in Senja when flows are deleted.

This is the standard pattern used by GitHub, Stripe, and Slack. With it, any Senja integration becomes zero-configuration: connect an API key, and webhook lifecycle is handled automatically.