Fix M3U write permission failure; add recently-downloaded section to dashboard

Playlist M3U regen now writes to a temp file and renames it into place,
so a stray wrong-owner leftover file (root:root, from pre-migration
host-cron runs) can't block nightly writes the way it did last night.
Dashboard now lists tracks added to the beets library in the last 24h
with playlist and format, sourced from a new beets_service.recently_added().

app/templates/dashboard.html

@@ -49,6 +49,25 @@
 </ul>
 <p><a href="/settings/credentials">Manage credentials &rarr;</a></p>
 
+<h2>Downloaded in the last 24 hours</h2>
+<div class="table-wrap scroll">
+  <table>
+    <thead><tr><th>Artist</th><th>Title</th><th>Playlist</th><th>Format</th></tr></thead>
+    <tbody>
+      {% for t in recent_tracks %}
+      <tr>
+        <td>{{ t.artist }}</td>
+        <td>{{ t.title }}</td>
+        <td class="muted">{{ t.grouping }}</td>
+        <td><span class="badge badge-muted">{{ t.format }}</span></td>
+      </tr>
+      {% else %}
+      <tr class="empty-row"><td colspan="4" class="muted">Nothing downloaded in the last 24 hours.</td></tr>
+      {% endfor %}
+    </tbody>
+  </table>
+</div>
+
 <h2>Recent job runs</h2>
 <div class="table-wrap scroll">
   <table>