The POI search endpoint (/api/maps/pois) returned 502 errors because:
1. OVERPASS_TIMEOUT_MS (12s) was shorter than mirror response times
(kumi.systems takes ~25.7s to first byte). Increased to 25s to match
the [timeout:20] query timeout.
2. The static User-Agent string was indistinguishable between instances,
making rate-limiting and throttling more likely. The new userAgent()
function appends the instance's APP_URL so each deployment identifies
itself uniquely, following Overpass API best practices.
544a76d2da