Fetch a list of podcasts with their meta-information

[illright]

Integrate with the Supabase backend to fetch a list of all podcasts with all of their meta information and the URL to the audio file. For now, just dump it as JSON somewhere.

Acceptance criteria:

• Every podcast from Supabase is fetched along with all of its meta information (we'll filter it out later)
• No private tokens are leaked in the client build
• Fetching works on both the main deployment and the preview deployments (on subdomains of cast-iu.pages.dev)
• The data is fetched in the format of proposed TS interfaces

Interfaces:

interface Podcast {
  id: number;
  title: string;
  author: string;
  coverUrl: string;
  episodes: Episode[];
}

interface Episode {
  id: number;
  title: string;
  duration: number;
  audioUrl: string;
  favorite?: boolean;  // omit this field for now
}

[illright]

I think it will be best to use GraphQL to be free about the resulting data format

[aabounegm]

Are the author and coverUrl supposed to be required/non-nullable? In #41 it seems from the screenshot that they can be null

[illright]

Yeah, they should be non-nullable. @VanishMax, could you please ensure they are?