Performance: Fix N+1 query pattern in arrivals and departures handler

[3rabiii]

Description

The arrivalsAndDeparturesForStopHandler in internal/restapi/arrivals_and_departure_for_stop.go currently suffers from a significant N+1 query problem.

Inside the main loop iterating over stopTimes, the code executes individual database queries for GetRoute and GetTrip for every single arrival.

Impact

For a busy stop with 50 upcoming arrivals, this results in 100+ separate database queries per single HTTP request. This causes high latency and unnecessary load on the database.

Goal

Refactor the handler to use Batch Fetching:

1. Collect all unique Route IDs and Trip IDs before the loop.
2. Fetch all required data in just two queries (GetRoutesByIDs and GetTripsByIDs).
3. Use in-memory maps for O(1) lookups inside the loop.