Journey handoff nodes now escalate to human operators

Journey flows can now hand customers off to human operators as a real operation.

• Handoff nodes set needs_human = true on the conversation scoped to the workspace, contact, and channel.
• Handoff metadata (target type, target id, priority, note) is stored in the journey run context for operator visibility.
• The inbound webhook no longer enqueues AI auto-reply when a conversation already needs a human.