[scrobbles] Refactor run time sec to be blank by default

vrobbler/apps/scrobbles/mixins.py

@@ -57,14 +57,20 @@ class ScrobblableMixin(TimeStampedModel):
 
     uuid = models.UUIDField(default=uuid4, editable=False, **BNULL)
     title = models.CharField(max_length=255, **BNULL)
-    run_time_seconds = models.IntegerField(default=900)
-    run_time_ticks = models.PositiveBigIntegerField(**BNULL)
+    base_run_time_seconds = models.IntegerField(**BNULL)
 
     genre = TaggableManager(through=ObjectWithGenres, blank=True)
 
     class Meta:
         abstract = True
 
+    @property
+    def run_time_seconds(self) -> int:
+        run_time = 900
+        if self.base_run_time_seconds:
+            run_time = self.base_run_time_seconds
+        return run_time
+
     @classmethod
     def is_long_play_media(cls) -> bool:
         return False