POST /v1/conversations should return a canonical web URL

[ckaznocha]

Context

An agent can start a fresh interactive conversation through the self-API and needs to provide the person a usable link to it.

Reproduction

1. Configure a QM deployment with a public web URL.
2. From an attended agent turn, call POST /v1/conversations with a text body.
3. Inspect the accepted response and ask the agent to share the new conversation.

Actual behavior

The response includes the new session record and queued turn but no canonical browser URL. The agent therefore has to infer a route and can return an API-shaped or otherwise invalid link.

Expected behavior

When a public web URL is configured, the successful response should include webUrl formed from that configured base and the returned session ID, for example <public-web-url>/s/<session-id>. The agent API catalog should direct agents to share that returned value verbatim rather than constructing a URL. When no public web URL is configured, the response may omit the field.

Proposed fix

At the successful spawn boundary, construct the link from the configured portal URL and session.id, add it to the response, and document the field in the POST /v1/conversations catalog entry.