Feature: GTFS Frequencies Phase 3 (API Integration & Handlers)

[3rabiii]

This issue tracks Phase 3 (the final phase) of the GTFS Frequencies implementation.
With Phase 1 (Storage/DB) and Phase 2 #582 (Core Logic & Helpers) already merged, this phase focuses entirely on wiring the frequency data into the API Handlers and Response Models.

Implementation

Part 1: Models & Core Handlers

• stop_time_schedule.go: Implement ScheduleFrequency struct and update slices.
• trip_details_handler.go: Populate Frequency, TripStatusForTripDetails.Frequency, and Schedule.Frequency.
• trips_helper.go (BuildTripSchedule): Populate Schedule.Frequency.
• arrivals_and_departure_for_stop.go: Populate Frequency and adjust predicted times based on headways for exact_times=0.
• trip_for_vehicle_handler.go: Populate frequency for the vehicle's active trip.

Part 2: Batch Queries & Schedule Handlers

• schedule_for_stop_handler.go: Build ScheduleFrequencies using the new struct. Expand stop times for exact_times=1.
• schedule_for_route_handler.go: Include frequency data per-trip-direction group.
• trips_for_route_handler.go: Populate *int64 headway field using batch query to avoid N+1.
• trips_for_location_handler.go: Populate using batch query and update time-window filtering logic for active frequency windows.

Testing Requirements

• Create createTestApiWithFrequencyData(t) fixture helper.
• Add subtests for trip_details_handler_test.go.
• Add subtests for trips_for_route_handler_test.go & trips_for_location_handler_test.go.
• Add subtests for schedule_for_stop_handler_test.go.
• Add subtests for arrivals_and_departure_for_stop_test.go.
• Verify standard regression suite passes perfectly with normal (non-frequency) data.

[3rabiii]

I'm working on this!