feat: allow re-downloading already-downloaded concerts

Replace disabled "Déjà téléchargé" with active "Re-télécharger" button
(ghost style to distinguish from first download).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

static/style.css

@@ -629,6 +629,8 @@ body {
 .btn-download:hover { opacity: 0.88; transform: translateY(-1px); }
 .btn-download:active { transform: none; }
 .btn-download:disabled { opacity: 0.5; cursor: default; transform: none; }
+.btn-download.btn-redownload { background: transparent; color: var(--text-muted); border: 1px solid var(--border); }
+.btn-download.btn-redownload:hover { color: var(--text); border-color: var(--text-muted); opacity: 1; }
 
 .btn-watch {
   color: var(--text-dim);