podkit: playlist URL to private podcast feed

One Go binary with fetch / tag / feed / serve / all. The stages run separately
on purpose: the download is the slow part and should not be repeated while
adjusting titles or the base URL.

Built from a shell + Python + Go sketch. The pieces that needed fixing on the
way, all of which produce a feed that loads wrong or not at all:

- Enclosure URLs were unescaped. Track titles routinely contain spaces and
  ampersands, which make both the URL and the XML invalid — the likeliest way
  for a feed to fail to load entirely. Only the final path segment is escaped;
  escaping the base would mangle scheme and host.
- Ordering came from filename sort, which is correct only while zero-padding is
  applied consistently. One unpadded file puts lecture 10 after lecture 1.
  Order now comes from the parsed track number.
- No pubDate and no itunes:type. Podcast clients default to newest-first, which
  starts a lecture course at the final lecture. Items now carry distinct
  ascending dates and the feed is marked serial.
- Missing channel description and link, which several clients reject outright.
- A struct field with no xml tag emitted a stray <EnclosureURL> element.
- guid had no isPermaLink="false", so a reader may treat it as a fetchable URL.
- Errors from ReadDir, Info and Marshal were discarded, so a missing directory
  produced an empty feed that looked like a working setup with nothing in it.
- os.Args indexed without a length check.

Beyond the sketch: a download archive so an interrupted series resumes, an
up-front check for yt-dlp and ffmpeg before the long download rather than a
post-processing failure after the bytes are on disk, non-audio files excluded
by extension so cover art and .part files cannot become episodes, and a serve
command with token gating that 404s rather than 401s.

Found by the first live run against a LibriVox collection: sources whose own
titles begin with a track number came out as "01 - 01 - April.mp3", because the
output template prepends the playlist index regardless. Collapsed, but only
when the two numbers agree — "01 - 1964 - Lecture" is a year and "05 - 12 Angry
Men" is a title, and neither is a repeat.

47 tests. Verified end to end against public-domain audio: downloaded,
converted, tagged, served, and the delivered file is a playable 128kbps MP3
with correct ID3 title/artist/album/track frames.

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