Launch Segment

Prepare a segment for launch: apply budget (+ optional target_url).

The agent session itself is still created by the client via POST /api/agent-chat/sessions — we don't want to spawn a Claude CLI subprocess from this request. Keeping the split lets the client flow stay unchanged while this endpoint takes care of the one-shot writes (budget + target_url) that used to leak across the 2-3 calls of onboarding/MiniOnboarding.

target_url is not required here — discovery/research runs fine without it. Outreach workers must gate on a non-empty value separately; the UI gates the Launch button so most callers will supply it.