Expose assigned room ID from committee endpoint

Extend `GET /api/committees/{id}` so its response includes the assigned room ID when a room currently references that committee, without duplicating room assignment state. Preserve existing committee response behavior and document the optional field in OpenAPI. Acceptance criteria: - A committee with an assigned default room returns that room's ID as `room_id`. - A committee without a default room returns `room_id: null`. - Room managers can assign or clear the default from cloud-dashboard. - A committee can only default to a room assigned to its municipality; deleting that room clears the default. - Existing not-found and authorization behavior remains unchanged. - Regression tests and cloud-router validation pass. Decision: store an explicit nullable default-room assignment on each committee. Expose it as `room_id`, allow authenticated room managers to assign or clear it, restrict assignments to rooms in the committee's municipality, clear it automatically if the room is deleted, and provide assignment UI in cloud-dashboard. Pull request: #125 https://git.kaukus.no/Kaukus/streaming-engines/pulls/125