Remove the in-memory cache from geocode/nominatim.go (cacheTTL, cacheMu,
cachedLoc, cacheExpiry) so CityCenter always makes a live request when called.
Call frequency is now controlled by the caller rather than a time-based TTL.
In the poll goroutine, log the raw GPS coordinates on every cycle. Track the
last geocoded position and only call CityCenter on the first fix or when the
device has moved more than 5 miles (haversine). WebDAV snapshots are written
every poll cycle using the cached city centroid with fresh weather data.
