Watch the cheapest price for a whole destination search

Runs priceWatch.createSearchWatch. Tracks the cheapest hotel for destination and the stay, rather than one hotel. Send currentPrice (the cheapest price on screen, a stay total) when creating from live results so it becomes the baseline; targetPrice must be below it. source defaults to saved_page; native search results should send search_results. 400 for past or reversed dates, or when the account already tracks the maximum number of destinations; 409 when this destination and stay are already tracked. Returns { watch }, the same shape the destination entries of GET /v1/price-watches carry.