Editors can add Listen on links to an episode; the podcast's links show when it has none

Problem/Motivation

Since #3622954, "Listen on" platform links live on the podcast only. An episode page can link to the show on Spotify or Apple Podcasts, but not to that episode, so editors cannot point listeners at the episode itself.

Steps to reproduce

1. Apply the Varbase Podcasts Base recipe and create a podcast with "Listen on" links.
2. Edit one of its episodes as a Content editor. There is no "Listen on" field.

Proposed resolution

• Add the existing field_listen_links storage to Podcast episode, in the General tab. Its help text says to leave it empty to use the podcast's links.
• The listen_on display of the podcast_episodes view shows the episode's own links, and falls back to the podcast's links when it has none.
• Tests: a new 03-07-episode-listen-on.feature covering both cases, a "Listen on" check on the episode form, and one fixture episode with its own Spotify link.

Related: #3627726, where the RightUp theme renders these links.

Remaining tasks

• ✅ File an issue
• ✅ Addition/Change/Update/Fix (MR !15)
• ✅ Testing to ensure no regression
• ✅ Automated unit/functional testing coverage
• ➖ Developer Documentation support
• ➖ User Guide Documentation support
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ❌ Review with the product owner
• ✅ Update Release Notes
• ✅ Release (2.0.0)

User interface changes

• The Podcast episode form gains an optional "Listen on" field in the General tab.

API changes

• N/A

Data model changes

• A field_listen_links field on Podcast episode, reusing the existing field storage.

Release notes snippet

• Editors can add "Listen on" links to an episode. The podcast's links show when an episode has none.

AI-Generated: Yes