Tracking: restore functionality broken by Spotify API changes

Spotify's API changes broke a large part of psst. Several developer Web API endpoints were retired and the api-partner (pathfinder GraphQL) began rejecting psst's OAuth developer token. This issue tracks the status of restoring or replacing each affected area.

General:

• partner_auth() Login5 bearer + client-token for api-partner (#745)
• Better error handling / display for pathfinder GraphQL

Artist page:

• Restore discography (#746)
• Restore info & related artists (#747)
• (Add a "Load more" button for discography)
• Remove profile.externalLinks, field gone

Home page:

• Restore home page (#745)
• Make generated ("algorithmic") playlists fetchable again (#754)
• Fix intermittent missing field 'data' on startup

Shows / Podcasts:

• Make show/podcasts episodes load again (#755)

Songs:

• Add "Song Radio" (related to generated playlists, would also add #189)
• Remove deprecated "Show Similar Tracks", endpoint gone

Start with the unchecked checklist items and linked issues #754 and #755, then inspect the affected Spotify Web API and api-partner (pathfinder GraphQL) flows. Done means the selected endpoint or page behavior is restored, deprecated fields and endpoints are removed or handled, and the affected functionality can be verified.