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/js/backups.js

@@ -213,6 +213,7 @@ function renderMainView() {
 
     '<div class="backup-footer">' +
     '<span class="backup-footer-hint">Backups crees lors de la sauvegarde manuelle (Ctrl+S)</span>' +
+    '<button class="backup-footer-manage-btn" id="backup-manage-btn">Gerer tous les backups</button>' +
     '</div>';
 
   setBody(html);
@@ -249,6 +250,14 @@ function bindEvents() {
   }
 
   var modeBtn = document.getElementById('backup-diff-mode-btn');
+  var manageBtn = document.getElementById('backup-manage-btn');
+  if (manageBtn) {
+    manageBtn.addEventListener('click', function() {
+      close();
+      import('./backup-manager.js').then(function(m) { m.openBackupManager(); });
+    });
+  }
+
   if (modeBtn) {
     modeBtn.addEventListener('click', function() {
       state.diffMode = state.diffMode === 'sidebyside' ? 'unified' : 'sidebyside';