feat: add per-song MP3 download buttons to admin request page

Adds an admin download route (/admin/request/<rid>/download-song/<version>)
and Download buttons next to Version A and Version B in section 2 (Songs),
shown only when the MP3 exists on disk. Bumps version to 0.9.2.

app.py

@@ -922,6 +922,24 @@ def admin_download_stems(rid):
     return send_file(buf, mimetype='application/zip', as_attachment=True, download_name=download_name)
 
 
+@app.route('/admin/request/<int:rid>/download-song/<version>')
+def admin_download_song(rid, version):
+    """Download a single song MP3 (Version A or B) for a request."""
+    redir = require_admin()
+    if redir:
+        return redir
+    req = get_request_by_id(rid)
+    if not req:
+        abort(404)
+    if version not in ('a', 'b'):
+        abort(404)
+    path = req.get('song_a_path') if version == 'a' else req.get('song_b_path')
+    if not path or not Path(path).exists():
+        flash('Song file not found.', 'error')
+        return redirect(url_for('admin_request', rid=rid))
+    return send_file(path, as_attachment=True, download_name=Path(path).name)
+
+
 # ---------------------------------------------------------------------------
 # Admin routes
 # ---------------------------------------------------------------------------