[books] Calc run time from pages on save

vrobbler/apps/books/models.py

@@ -179,6 +179,13 @@ class Book(LongPlayScrobblableMixin):
             return f"{self.title} - Volume {self.volume_number}"
         return f"{self.title}"
 
+    def save(self, *args, **kwargs):
+        if self.pages:
+            self.base_run_time_seconds = int(self.pages) * int(
+                self.AVG_PAGE_READING_SECONDS
+            )
+        super().save(*args, **kwargs)
+
     @property
     def subtitle(self):
         return f" by {self.author}"
@@ -413,11 +420,6 @@ class Book(LongPlayScrobblableMixin):
                     fname = f"{self.title}_{self.uuid}.jpg"
                     self.cover.save(fname, ContentFile(r.content), save=True)
 
-            if self.pages:
-                self.base_run_time_seconds = int(self.pages) * int(
-                    self.AVG_PAGE_READING_SECONDS
-                )
-
             self.save()
 
     def fix_authors_metadata(self, openlibrary_author_id):