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/index.html

@@ -1284,10 +1284,29 @@
                                     value="20"
                                 />
                                 <span class="config-hint"
-                                    >Entre 5 et 100 fichiers (sauvegardé sur le
+                                    >Entre 5 et 100 fichiers (sauvegarde sur le
                                     serveur)</span
                                 >
                             </div>
+                            <div class="config-row">
+                                <label
+                                    class="config-label"
+                                    for="cfg-max-backups"
+                                    >Max backups par fichier</label
+                                >
+                                <input
+                                    type="number"
+                                    id="cfg-max-backups"
+                                    class="config-input config-input--num"
+                                    min="1"
+                                    max="50"
+                                    step="1"
+                                    value="10"
+                                />
+                                <span class="config-hint"
+                                    >Nombre maximum de versions backup conservees par fichier (1-50)</span
+                                >
+                            </div>
                         </section>
 
                         <!-- Performance Settings - Backend -->