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.

1 files changed, 91 insertions, 0 deletions

diff --git a/internal/serve/serve.go b/internal/serve/serve.go
new file mode 100644
index 0000000..0b3b340
--- /dev/null
+++ b/internal/serve/serve.go
@@ -0,0 +1,91 @@
+// Package serve hosts the audio directory and its feed over HTTP.
+package serve
+
+import (
+	"crypto/rand"
+	"crypto/subtle"
+	"encoding/hex"
+	"fmt"
+	"net/http"
+	"path"
+	"strings"
+)
+
+// Options configure the server.
+type Options struct {
+	Dir  string
+	Addr string
+	// Token, if set, must appear in every request as ?k=<token> or an
+	// Authorization: Bearer header.
+	//
+	// "Private" here means unlisted-and-unguessable, not authenticated in any
+	// serious sense: podcast clients do not do interactive login, so a shared
+	// secret in the URL is the mechanism that actually works with them. Anyone
+	// holding the URL holds the access.
+	Token string
+}
+
+// NewToken returns a random token suitable for Options.Token.
+func NewToken() (string, error) {
+	b := make([]byte, 16)
+	if _, err := rand.Read(b); err != nil {
+		return "", fmt.Errorf("generating token: %w", err)
+	}
+	return hex.EncodeToString(b), nil
+}
+
+// Handler builds the HTTP handler: the feed at /feed.xml, audio under /audio/.
+func Handler(opts Options) http.Handler {
+	mux := http.NewServeMux()
+
+	files := http.FileServer(http.Dir(opts.Dir))
+	mux.Handle("/audio/", http.StripPrefix("/audio/", files))
+
+	mux.HandleFunc("/feed.xml", func(w http.ResponseWriter, r *http.Request) {
+		http.ServeFile(w, r, path.Join(opts.Dir, "feed.xml"))
+	})
+
+	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/" {
+			http.NotFound(w, r)
+			return
+		}
+		w.Header().Set("Content-Type", "text/plain; charset=utf-8")
+		fmt.Fprintln(w, "podkit — subscribe your podcast app to /feed.xml")
+	})
+
+	return withToken(opts.Token, mux)
+}
+
+// withToken gates every request on a shared secret.
+func withToken(token string, next http.Handler) http.Handler {
+	if token == "" {
+		return next
+	}
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if !tokenOK(r, token) {
+			// 404 rather than 401: a 401 confirms something is here worth
+			// guessing at, and no podcast client is going to act on the
+			// challenge anyway.
+			http.NotFound(w, r)
+			return
+		}
+		next.ServeHTTP(w, r)
+	})
+}
+
+func tokenOK(r *http.Request, want string) bool {
+	if got := r.URL.Query().Get("k"); got != "" && constantTimeEqual(got, want) {
+		return true
+	}
+	auth := r.Header.Get("Authorization")
+	if strings.HasPrefix(auth, "Bearer ") {
+		return constantTimeEqual(strings.TrimPrefix(auth, "Bearer "), want)
+	}
+	return false
+}
+
+// constantTimeEqual avoids leaking the token through response timing.
+func constantTimeEqual(a, b string) bool {
+	return subtle.ConstantTimeCompare([]byte(a), []byte(b)) == 1
+}