Add Google Directions API as alternative transit backend

[Eddylhb]

Feature Request: Google Directions API as Public Transit Backend

Problem

The current public transit feature relies solely on Transitous, which has very limited GTFS coverage in many regions — including Japan, China, and most of Asia. Searching for routes between major subway stations (e.g., Nakanoshima → Temmabashi in Osaka) returns "No routes found" because the stops simply don't exist in Transitous's database.

Proposed Solution

Add an optional Google Maps Directions API (mode=transit) backend for public transit routing. TREK already supports Google Places API, Maps JavaScript API, and Geocoding API via the user's own API key in Admin → Settings → API Keys — the same key can be reused.

How It Would Work

1. Admin → Settings → Transit Provider: [Transitous] or [Google Transit]
2. If Google Transit is selected and a valid API key is configured, transit searches use the Directions API
3. Fall back to Transitous if no Google key is set

Why This Matters

• Google has comprehensive transit data for 80+ countries, far exceeding Transitous coverage
• Users already have Google API keys configured in TREK (no additional setup)
• Google's free tier ($200/month) easily covers personal/household transit queries
• This makes TREK actually usable for trip planning across all regions, not just Europe

Technical Notes

• Endpoint: https://maps.googleapis.com/maps/api/directions/json?mode=transit
• Already covered by existing API key restrictions
• Response format is well-documented and stable

Thanks for an amazing tool — this would be the missing piece for non-European travelers!