Add the submission pipeline and the review loop

Steps 3 and 4 of the build order. A member can now upload a song, watch it
convert, publish it, and review what everyone else has published.

Pipeline:
- ffprobe reads tags synchronously at submit so prefill never races typing;
  ffmpeg converts to Opus in the background, two at a time
- ffmpeg succeeding is the validation — no container sniffing
- publish moves the file inside the transaction, so a song row and its .ogg
  appear together or neither does
- five submissions per rolling 24h, failures excluded

Reviews and the reveal rule:
- the queue is unreviewed songs only, oldest first, never your own
- other people's reviews and the average are withheld in the query, not the
  template — a hidden average is never sent
- 30 minutes to edit or delete your own review, enforced in the WHERE clause
- deleting the last review unlocks the song for its submitter again

The waiting page has one button: the metadata form autosaves after a pause in
typing, and Julkaise submits it and publishes in the same request, so nothing
is lost without JS.

Genres store an English code and render a Finnish label.

templates/songs.html

@@ -0,0 +1,16 @@
+{{define "content"}}
+<h1>Kappaleet</h1>
+
+{{if .Data.Items}}
+  <table>
+    <thead><tr><th>Kappale</th><th>Genre</th><th>Kesto</th><th>Pisteet</th></tr></thead>
+    <tbody>
+      {{range .Data.Items}}{{template "songrow" .}}{{end}}
+    </tbody>
+  </table>
+  {{with .Data.NextCursor}}<p><a href="/songs?cursor={{.}}">Vanhempia →</a></p>{{end}}
+{{else}}
+  <p class="empty">Yhtään kappaletta ei ole vielä julkaistu.</p>
+  <p><a href="/submit">Lähetä ensimmäinen</a>.</p>
+{{end}}
+{{end}}