Fix Faktiskā showing blank temperatures right after each new hour starts

queryLatestTemperatures picked the single most-recent row per city with no
regard for whether that row's tempAvg was actually populated yet. The
open-data source publishes fields on different schedules within the same
hour (snow lands well before the hourly temperature aggregate), so the
freshest row for a city routinely has a real dateTime but a still-null
tempAvg while the previous hour's row has good data. Confirmed live on both
Rocky and the VPS: every city's newest row had tempAvg null (but a real
snowAvg=0), one hour after the actual last real reading.

Fixed by requiring tempAvg IS NOT NULL in the WHERE clause, so DISTINCT ON's
"most recent" pick skips a fresh partial row and finds the newest row that
actually has a value. Checked before changing: this is the only caller of
queryLatestTemperatures (Faktiskā's fixed-station lookup); a city with zero
real temperature history ever now returns no row instead of a null-valued
one, which the frontend already treats identically to a present-but-null
observation (falls through to the same "Nav datu" missing state either
way) -- confirmed via MapGraphics.tsx before shipping, not assumed.

src/main/scala/db/PostgresService.scala

@@ -29,10 +29,17 @@ object PostgresService {
 
 class PostgresService(transactor: Transactor[IO], log: Logger[IO]) {
   def queryLatestTemperatures(cities: NonEmptyList[String]): IO[List[(String, LocalDateTime, Option[Double])]] = {
+    // The open-data source publishes fields on different schedules within
+    // the same hour (e.g. snow arrives well before the hourly temperature
+    // aggregate), so the single newest row for a city can have a real
+    // dateTime but a still-null tempAvg. Skip straight to DISTINCT ON's
+    // "most recent" pick by requiring tempAvg itself to be present, so a
+    // fresh partial row never hides an older row that actually has data.
     val query =
       fr"SELECT DISTINCT ON (city) city, dateTime, tempAvg" ++
         fr" FROM weather" ++
         fr" WHERE " ++ Fragments.in(fr"city", cities) ++
+        fr" AND tempAvg IS NOT NULL" ++
         fr" ORDER BY city, dateTime DESC"
 
     query.query[(String, LocalDateTime, Option[Double])]