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/config.js

@@ -665,6 +665,7 @@ async function loadConfigFields() {
     _setField("cfg-tag-boost", data.tag_boost);
     _setField("cfg-prefix-exp", data.prefix_max_expansions);
     _setField("cfg-recent-limit", data.recent_files_limit || 20);
+    _setField("cfg-max-backups", data.max_backups_per_file || 10);
     // Watcher config
     _setCheckbox("cfg-watcher-enabled", data.watcher_enabled !== false);
     _setCheckbox("cfg-watcher-polling", data.watcher_use_polling === true);
@@ -716,6 +717,7 @@ async function saveBackendConfig() {
     tag_boost: _getFieldNum("cfg-tag-boost", 2.0),
     prefix_max_expansions: _getFieldNum("cfg-prefix-exp", 50),
     recent_files_limit: _getFieldNum("cfg-recent-limit", 20),
+    max_backups_per_file: _getFieldNum("cfg-max-backups", 10),
     watcher_enabled: _getCheckbox("cfg-watcher-enabled"),
     watcher_use_polling: _getCheckbox("cfg-watcher-polling"),
     watcher_polling_interval: _getFieldNum("cfg-watcher-interval", 5.0),