Move ListeningRules to its own file and split listening tests by subject

The counted-listen tests lived in one catch-all UncountedListensTests
file. Each now sits next to the handler it covers, and the cleanup
handler gets its own test file.

backend/Core/Musify.Application/Tracks/ListeningRules.cs

@@ -0,0 +1,14 @@
+namespace Musify.Application.Tracks
+{
+    public static class ListeningRules
+    {
+        public const double CountedThresholdSeconds = 30;
+        public const double CountedThresholdFraction = 0.5;
+        public const double DurationToleranceSeconds = 2;
+        public const double ClockToleranceSeconds = 5;
+        public const int UncountedRetentionDays = 8;
+
+        public static double CountedThreshold(double durationSeconds) =>
+            Math.Min(CountedThresholdSeconds, durationSeconds * CountedThresholdFraction);
+    }
+}

backend/Core/Musify.Application/Tracks/RecordListeningProgress.cs

@@ -11,18 +11,6 @@ namespace Musify.Application.Tracks
         double PlayedSeconds)
         : ICommand<ErrorOr<Success>>;
 
-    public static class ListeningRules
-    {
-        public const double CountedThresholdSeconds = 30;
-        public const double CountedThresholdFraction = 0.5;
-        public const double DurationToleranceSeconds = 2;
-        public const double ClockToleranceSeconds = 5;
-        public const int UncountedRetentionDays = 8;
-
-        public static double CountedThreshold(double durationSeconds) =>
-            Math.Min(CountedThresholdSeconds, durationSeconds * CountedThresholdFraction);
-    }
-
     public class RecordListeningProgressCommandHandler(IDatabase database)
         : ICommandHandler<RecordListeningProgressCommand, ErrorOr<Success>>
     {