Update Itinerary

Partial update of a route: delivery channels and the search window.

The NIGHTS are not editable — their order and adjacency are validated as a whole at create time (walkable hops, a trailhead on night 1), so changing one in isolation could leave a route the provider would refuse to book. Changing those means creating a new route.

The window is different: shifting or widening it re-derives every leg's range mechanically and cannot invalidate the sequence. Widening it is also the single most useful edit — more candidate start days is more chances to fire — which is exactly why it is editable on an ordinary alert too.

party_size and tent_pads are editable for the same reason they are on an ordinary alert — and for routes they are not cosmetic: both feed the capacity gate (a night's zone must have max(tent_pads, party_size, 1) units free). The handler mirrors any change onto every leg and re-baselines the route so a looser requirement can fire again instead of staying suppressed by the prior signature.