old_contents: from django.db import models
  from django.contrib.auth.models import User
  from mygpo.api.models import Podcast
  
  class PodcastPublisher(models.Model):
      user = models.ForeignKey(User)
      podcast = models.ForeignKey(Podcast)
  
      class Meta:
          db_table = 'publisher'
  
new_contents: from django.db import models
  from django.contrib.auth.models import User
  from mygpo.api.models import Podcast
  
  class PodcastPublisher(models.Model):
      user = models.ForeignKey(User)
      podcast = models.ForeignKey(Podcast)
  
      class Meta:
          unique_together = ('user', 'podcast')
          db_table = 'publisher'
  
subject: add missing unique constraint for PodcastPublisher