Build out the podcast: live feed, subscribe surface, episode pages

- /podcast/feed.xml: valid podcast RSS with iTunes tags and audio
  enclosures, populated only by episodes that have real audio
- Podcast index: subscribe row with the feed address; episode rows gain
  guests and duration; empty state points at the live feed
- Episode pages: guests/duration line, framed player with download link
- Front page: compact podcast rail appears once the first episode exists
- Nav shows Podcast when the feed is configured; footer links the feed

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/content/episodes/episode-template.md

@@ -11,9 +11,11 @@ status: developing
 draft: true
 duration: "00:00"
 guests: []
+# audioUrl: https://…/episode-001.mp3   (durable location only)
+# audioBytes: 0                          (file size in bytes, for the feed enclosure)
 ---
 
-> Draft episode template. Add the audio URL in frontmatter only after the durable feed location is known.
+> Draft episode template. Add the audio URL in frontmatter only after the durable feed location is known. Episodes without an audio URL never enter the podcast feed.
 
 ## Listen