feat: backup management page + max backups config + auto-cleanup

- Config: max_backups_per_file (default 10) in config.json + UI field
- Auto-cleanup: _backup_file deletes oldest backups when limit exceeded
- New endpoints: GET /api/backups (list all), POST /api/backups/delete,
  POST /api/backups/purge (by file or vault)
- New module: backup-manager.js - full-page view with filter, grouped by
  vault/file, individual delete, per-file purge, stats
- Link from backup viewer footer: 'Gerer tous les backups'

frontend/style.css

@@ -7499,3 +7499,119 @@ body.popup-mode .content-area {
   width: 14px;
   height: 14px;
 }
+
+/* ── Backup Manager ── */
+.backup-manager {
+  padding: 16px 24px;
+  max-width: 1000px;
+  margin: 0 auto;
+}
+
+.backup-manager-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  margin-bottom: 12px;
+  gap: 12px;
+}
+
+.backup-manager-header h2 {
+  margin: 0;
+  font-size: 1.1rem;
+  display: flex;
+  align-items: center;
+  gap: 8px;
+}
+
+.backup-manager-actions {
+  display: flex;
+  gap: 6px;
+}
+
+.backup-manager-filter {
+  padding: 6px 10px;
+  border: 1px solid var(--border);
+  border-radius: 6px;
+  background: var(--bg-primary);
+  color: var(--text-primary);
+  font-size: 0.8rem;
+  min-width: 200px;
+}
+
+.backup-manager-btn {
+  width: 32px; height: 32px;
+  border: 1px solid var(--border);
+  border-radius: 6px;
+  background: transparent;
+  color: var(--text-secondary);
+  cursor: pointer;
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+}
+
+.backup-manager-btn:hover { color: var(--accent); border-color: var(--accent); }
+
+.backup-manager-stats { display: flex; gap: 16px; margin-bottom: 14px; }
+.bm-stat { font-size: 0.8rem; color: var(--text-muted); }
+
+.bm-vault-group { margin-bottom: 16px; }
+.bm-vault-name {
+  font-size: 0.85rem; font-weight: 600; margin: 0 0 6px 0;
+  display: flex; align-items: center; gap: 6px;
+}
+
+.bm-file-group {
+  border: 1px solid var(--border); border-radius: 6px;
+  margin-bottom: 8px; overflow: hidden;
+}
+
+.bm-file-header {
+  display: flex; align-items: center; gap: 12px;
+  padding: 8px 12px; background: var(--bg-secondary);
+  border-bottom: 1px solid var(--border);
+}
+
+.bm-file-name { font-size: 0.8rem; font-weight: 500; display: flex; align-items: center; gap: 4px; }
+.bm-file-meta { font-size: 0.7rem; color: var(--text-muted); flex: 1; }
+
+.bm-file-purge {
+  width: 28px; height: 28px; border: 1px solid var(--border); border-radius: 4px;
+  background: transparent; color: var(--text-muted); cursor: pointer;
+  display: flex; align-items: center; justify-content: center;
+}
+.bm-file-purge:hover { color: var(--danger); border-color: var(--danger); }
+
+.bm-backup-list { padding: 2px 0; }
+
+.bm-backup-item {
+  display: flex; align-items: center; gap: 12px;
+  padding: 4px 12px; font-size: 0.75rem;
+}
+.bm-backup-item:hover { background: var(--bg-secondary); }
+
+.bm-backup-date { color: var(--text-secondary); min-width: 130px; font-family: var(--font-mono); }
+.bm-backup-size { color: var(--text-muted); flex: 1; }
+
+.bm-backup-delete {
+  width: 22px; height: 22px; border: none; border-radius: 4px;
+  background: transparent; color: var(--text-muted); cursor: pointer;
+  display: flex; align-items: center; justify-content: center;
+  opacity: 0;
+}
+.bm-backup-item:hover .bm-backup-delete { opacity: 1; }
+.bm-backup-delete:hover { color: var(--danger); background: rgba(239,68,68,0.1); }
+
+.backup-footer-manage-btn {
+  background: none; border: 1px solid var(--border);
+  border-radius: 4px; color: var(--accent);
+  font-size: 0.7rem; cursor: pointer; padding: 3px 10px;
+  transition: all 150ms ease;
+}
+.backup-footer-manage-btn:hover { background: color-mix(in srgb, var(--accent) 10%, transparent); }
+
+.backup-manager-empty,
+.backup-manager-error {
+  text-align: center; padding: 2rem; color: var(--text-muted); font-size: 0.85rem;
+}
+.backup-manager-error { color: var(--danger); }