fix(tournament): replace scala.util.Random singleton with UUID for native image

GraalVM native image fails when scala.util.Random companion object (a static
singleton with cached seed) is reachable from the image heap. UUID.randomUUID()
is always runtime-initialized and safe.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

modules/tournament/src/main/scala/de/nowchess/tournament/service/TournamentService.scala

@@ -49,7 +49,7 @@ class TournamentService:
   @Transactional
   def create(createdBy: String, form: CreateTournamentForm): Tournament =
     val t = new Tournament()
-    t.id = scala.util.Random.alphanumeric.take(6).mkString
+    t.id = java.util.UUID.randomUUID().toString.replace("-", "").take(8)
     t.fullName = form.name
     t.nbRounds = form.nbRounds
     t.clockLimit = form.clockLimit