feat: module toggles in POS config and Instance Manager

- Add GET/PUT /pos/api/config/modules endpoints in POS config_bp.py
- Update sidebar.js to filter nav items based on enabled modules
- Add Modules section to POS config.html with toggles for WhatsApp, Marketplace, MercadoLibre
- Add module load/save logic to POS config.js
- Preload modules in app-init.js for sidebar caching

- Add tenant module management to Instance Manager
  - get_tenant_modules / update_tenant_modules in tenant_service.py
  - GET/PUT /api/tenants/<id>/modules endpoints in tenants_bp.py
  - Add modules modal to manager index.html
  - Add module editing UI and logic to manager.js
  - Add toggle-switch CSS to manager.css

pos/blueprints/config_bp.py

@@ -579,6 +579,58 @@ def update_whatsapp_config():
     return jsonify({'message': 'WhatsApp configuration updated'})
 
 
+@config_bp.route('/modules', methods=['GET'])
+@require_auth('config.view')
+def get_modules():
+    """Get enabled modules for this tenant."""
+    conn = get_tenant_conn(g.tenant_id)
+    cur = conn.cursor()
+    cur.execute("SELECT key, value FROM tenant_config WHERE key LIKE 'module_%'")
+    rows = {row[0]: row[1] for row in cur.fetchall()}
+    cur.close()
+    conn.close()
+
+    def _bool(key):
+        return rows.get(key, 'true').lower() == 'true'
+
+    return jsonify({
+        'whatsapp': _bool('module_whatsapp'),
+        'marketplace': _bool('module_marketplace'),
+        'meli': _bool('module_meli'),
+    })
+
+
+@config_bp.route('/modules', methods=['PUT'])
+@require_auth('config.edit')
+def update_modules():
+    """Update enabled modules for this tenant."""
+    data = request.get_json() or {}
+    conn = get_tenant_conn(g.tenant_id)
+    cur = conn.cursor()
+
+    settings = {
+        'module_whatsapp': 'true' if data.get('whatsapp') else 'false',
+        'module_marketplace': 'true' if data.get('marketplace') else 'false',
+        'module_meli': 'true' if data.get('meli') else 'false',
+    }
+
+    for key, value in settings.items():
+        cur.execute("""
+            INSERT INTO tenant_config (key, value) VALUES (%s, %s)
+            ON CONFLICT (key) DO UPDATE SET value = EXCLUDED.value
+        """, (key, value))
+
+    conn.commit()
+    cur.close()
+    conn.close()
+
+    return jsonify({'message': 'Modules updated', 'modules': {
+        'whatsapp': data.get('whatsapp'),
+        'marketplace': data.get('marketplace'),
+        'meli': data.get('meli'),
+    }})
+
+
 @config_bp.route('/onboarding-status', methods=['GET'])
 @require_auth('pos.view')
 def get_onboarding_status():