Add city-level tracking to geo stats

- Update update-geo-stats to accept optional city parameter
- Update get-cached-geo to cache and return city along with country
- Update collect-geo-stats-for-mount and collect-geo-stats-from-web-listeners
  to track by country+city
- Revert migration to keep UNIQUE(date, country_code, city) constraint

migrations/002-listener-statistics.sql

@@ -72,7 +72,7 @@ CREATE TABLE IF NOT EXISTS listener_geo_stats (
     city VARCHAR(100),
     listener_count INTEGER DEFAULT 0,
     listen_minutes INTEGER DEFAULT 0,
-    UNIQUE(date, country_code)
+    UNIQUE(date, country_code, city)
 );
 
 CREATE INDEX IF NOT EXISTS idx_geo_stats_date ON listener_geo_stats(date);