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.

templates/submission.html

@@ -3,9 +3,17 @@
   <p class="{{if .Failed}}error{{end}}"><strong>{{.Label}}</strong></p>
   {{if .Failed}}
     {{with .StatusMsg}}<p class="muted small">{{.}}</p>{{end}}
-    <form method="post" action="/submit/{{.ID}}/discard">
-      <button type="submit">Poista lähetys</button>
-    </form>
+    <div class="actions">
+      {{if .CanRetry}}
+        <form method="post" action="/submit/{{.ID}}/retry">
+          <button type="submit">Yritä uudelleen</button>
+        </form>
+      {{end}}
+      <form method="post" action="/submit/{{.ID}}/discard">
+        <button type="submit" class="danger">Poista lähetys</button>
+      </form>
+    </div>
+    {{if not .CanRetry}}<p class="muted small">Lataa tiedosto uudelleen, jos haluat yrittää toisen kerran.</p>{{end}}
   {{else}}
     <!-- Outside the form and attached to it with form=, so one button both submits the metadata
          and publishes. Disabled until the audio has finished converting. -->