feat: Expand stops filters to include routes at stops

[sloanelybutsurely]

Adds fetch_routes_for_stops/1 and uses that to expand a stops: filter similar to the V3 API.

[digitalcora]

I may have missed or misunderstood something, but I was under the impression from previous discussions that re-implementing the API's "alert matching" logic within our cache was specifically something we wanted to avoid — with the reasoning that, in most cases where we fetch alerts, we already have the needed context of route type/route/stop/direction handy, and could pass those as explicit filters to the cache functions (which would be taken literally, not "expanded"). I don't disagree with this approach, just wondering how we arrived here.

EDIT: Taking the approach of being as compatible as possible with how the V3 API works, so we have greater confidence we're not changing the behavior of any existing uses of alert fetching in our app.

[digitalcora]

🚉 🔍 ✅