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/partials/player.html

@@ -0,0 +1,30 @@
+{{define "player"}}
+<audio controls preload="none" src="/audio/{{.ID}}" class="player"></audio>
+{{end}}
+
+{{define "songrow"}}
+<tr>
+  <td>
+    <a href="/songs/{{.ID}}">{{.Title}}</a>
+    <span class="muted"> — {{.Artist}}</span>
+    {{if .Own}}<span class="tag">oma</span>{{else if .Reviewed}}<span class="tag done">arvosteltu</span>{{end}}
+  </td>
+  <td class="nowrap">{{.GenreLabel}}</td>
+  <td class="nowrap">{{.Length}}</td>
+  <td class="nowrap">
+    {{if .Average}}<strong>{{score .Average}}</strong> <span class="muted">({{.ReviewCount}})</span>
+    {{else if .ReviewCount}}<span class="muted">{{.ReviewCount}} arvostelua</span>
+    {{else}}<span class="muted">—</span>{{end}}
+  </td>
+</tr>
+{{end}}
+
+{{define "scorefield"}}
+<label>Pisteet
+  <span class="scorerow">
+    <input type="range" name="score" min="1" max="100" value="{{.}}"
+           oninput="this.nextElementSibling.value = this.value">
+    <output>{{.}}</output>
+  </span>
+</label>
+{{end}}