fix: Don't crash the timetable page when the shuttle route isn't available

[joshlarson]

Scope

This should fix the fact that the Providence timetable on dev is unable to load right now.

Implementation

• Replace @routes_repo.get(shuttle_route_id) with a skeleton %Route{id: shuttle_route_id, type: 3}. We don't need any info about the shuttle route other than its route ID - the schedule query happens elsewhere (and gracefully handles a missing route).

Screenshots

How to test

• Clear your cache (Dotcom.Cache.Multilevel.flush in an IEx shell), and then look at the Providence timetable using dev or prod data.
• On this branch, it will load exactly what you'd expect
• On main, it'll crash