feat: MercadoLibre integration + inventory bulk publish + WhatsApp bridge fixes

- Add MercadoLibre OAuth, listings, orders, webhooks and category search
- New marketplace_external_bp.py, meli_service.py, marketplace_external_service.py
- New marketplace_external.html/js with ML management UI
- Inventory: bulk publish to ML with category autocomplete, listing type and shipping selectors
- Inventory: new .btn--meli styles, select/label CSS fixes
- WhatsApp bridge: rate limiting, 440/515/408 error handling, stale watchdog
- DB migration v3.4_meli_integration.sql for marketplace_listings, orders, sync_queue
- Add Celery tasks for ML sync and webhook processing
- Sidebar: MercadoLibre navigation link

pos/app.py

@@ -59,6 +59,9 @@ def create_app():
     from blueprints.marketplace_bp import marketplace_bp
     app.register_blueprint(marketplace_bp)
 
+    from blueprints.marketplace_external_bp import marketplace_ext_bp
+    app.register_blueprint(marketplace_ext_bp)
+
     from blueprints.peer_bp import peer_bp
     app.register_blueprint(peer_bp)
 
@@ -180,6 +183,14 @@ def create_app():
     def pos_marketplace():
         return render_template('marketplace.html')
 
+    @app.route('/pos/marketplace-external')
+    def pos_marketplace_external():
+        return render_template('marketplace_external.html')
+
+    @app.route('/pos/marketplace-external/callback')
+    def pos_marketplace_external_callback():
+        return render_template('marketplace_external.html')
+
     @app.route('/pos/static/<path:filename>')
     def pos_static(filename):
         return send_from_directory('static', filename)