Create Web Call
Create a browser-based web call.
Returns an access token that your frontend passes to the AgentPhone Web SDK
(`agentphone-web-sdk`) to start the call in the browser. The token
expires 30 seconds after creation.
Flow:
1. Validates the agent belongs to your account
2. Creates a web call and returns an access token
3. Your client calls `webClient.startCall({ accessToken })` to connect
4. Call lifecycle (transcripts, webhooks) works the same as phone calls

