refactor: optimize trip filtering and fix service date calculation

[3rabiii]

Context

This PR addressing suggestions from the previous review regarding arrivalsAndDeparturesForStopHandler.

Changes

• Fix serviceDate: Updated serviceDateMillis to use the calculated serviceMidnight instead of the raw request timestamp. This ensures the date correctly represents midnight of the service day.
• Optimize Filtering: Eliminated the expensive GetTripsByServiceID database query which loaded all trips into memory. Instead, stopTimes are now filtered directly using ServiceID (via an O(1) lookup set from activeServiceIDs).

Impact

• Corrects the semantic meaning of serviceDate in the response.
• Significantly reduces database load and memory usage by avoiding fetching all trips for active services.

@aaronbrethorst
fixes : #397

[aaronbrethorst]

Elegant simplification, Adel. Replacing a full database round-trip with an in-memory set lookup is exactly the right move here, and the serviceDate fix catches a subtle semantic bug.

What Changed

1. Eliminate GetTripsByServiceID query — Instead of fetching all trips for active service IDs from the database (potentially thousands of rows) just to build a trip ID filter set, the code now builds an activeServiceIDSet directly from the already-available activeServiceIDs slice and filters allStopTimes by st.ServiceID. This is semantically equivalent — a stop time is active iff its service is active — and avoids a costly DB round-trip plus the memory for the full trip result set.

2. Fix serviceDateMillis — Previously computed from params.Time.UnixMilli() (the raw request timestamp, e.g., 14:30:00). Now computed from serviceMidnight.UnixMilli(), which correctly represents midnight of the service day. This matches the semantic meaning of serviceDate in the OBA API response.

Review

No issues found. Both changes are correct, well-motivated, and result in a net reduction of 6 lines. The ServiceID field on GetStopTimesForStopInWindowRow (confirmed in query.sql.go:2643) makes the in-memory filtering straightforward. Existing tests all pass.

[3rabiii]

Hi Aaron, since this is approved, could we get it merged? Thanks!
@aaronbrethorst

[aaronbrethorst]

@3rabiii my bad, sorry!