Add the YouTube submission path

Step 5. A URL goes through the same pipeline as an upload — it just gains a
download step and a source_url.

- The host allowlist is checked on the parsed hostname before yt-dlp is
  invoked, so lookalikes and userinfo tricks are refused too
- yt-dlp -J reads metadata synchronously with a 15s timeout; a timeout leaves
  the fields blank rather than failing the submission
- Over-long tracks are refused from that metadata, before a byte is downloaded
- Failed URL submissions offer Yritä uudelleen with the typed text intact;
  uploads cannot retry, so they offer re-upload

Prefill takes track then title, and artist then creator then uploader, and
leaves a field blank rather than inventing one. testdata/ytdlp-noose.json is a
real dump of an ordinary upload, which has none of the music fields.

Also fixes a URL-only submit being blocked by the file input's required
attribute — HTML cannot express "one of these two", so the server says it.

The image now takes yt-dlp from Alpine 3.24 instead of pip, which drops
python3 and pip entirely; see decision 19.

main.go

@@ -152,6 +152,7 @@ func (a *app) memberMux() *http.ServeMux {
 	mux.HandleFunc("GET /submit/{id}/status", a.requireMember(a.submissionStatus))
 	mux.HandleFunc("POST /submit/{id}", a.requireMember(a.saveSubmission))
 	mux.HandleFunc("POST /submit/{id}/publish", a.requireMember(a.publish))
+	mux.HandleFunc("POST /submit/{id}/retry", a.requireMember(a.retry))
 	mux.HandleFunc("POST /submit/{id}/discard", a.requireMember(a.discard))
 	return mux
 }