fix(files): list a hotel booking's files and files pinned to a stop on the place card

A hotel booking reaches its place through the stay, not its own place_id, so
the card listed the booking but not its confirmation. Links to a stop, which
MCP's link_trip_file and plugins can make, were never returned by the file
list at all; it now carries linked_assignment_ids. Without files the row no
longer opens to nothing.

client/src/types.ts

@@ -105,6 +105,8 @@ export interface TripFile {
   reservation_title?: string
   linked_reservation_ids?: (number | null)[]
   linked_place_ids?: (number | null)[]
+  /** Stops in the plan the file is pinned to (MCP's link_trip_file, plugins). */
+  linked_assignment_ids?: (number | null)[]
   /** Served download path — always present on list/create/update responses (formatFile). */
   url: string
 }