Create Deal Link

Attach a proposal / invoice / booking to a deal.

Creates a canonical assignment for an unassigned item, or swaps in a different target. The kanban resolver picks the most-recent linked entity, so creating a new link with the user's chosen target automatically wins over older links of the same type. Removing stale links is a separate DELETE endpoint.

Delegates to services.deal_link_service.link_deal_item — the same use case the chat agent's link_deal_item tool calls, so org-scoping, idempotency, the archived-deal refusal, and deal.item_linked emission live in exactly one place.