Add allow_zero_price_sales toggle: config UI, POS enforcement and backend validation

pos/blueprints/config_bp.py

@@ -1130,6 +1130,39 @@ def update_counter_remission_config():
     return jsonify({'enabled': enabled == 'true'})
 
 
+@config_bp.route('/sales-settings', methods=['GET'])
+@require_auth('pos.view')
+def get_sales_settings():
+    """Get sales-related settings (zero-price sales, etc.)."""
+    conn = get_tenant_conn(g.tenant_id)
+    cur = conn.cursor()
+    cur.execute("SELECT value FROM tenant_config WHERE key = 'allow_zero_price_sales'")
+    row = cur.fetchone()
+    cur.close(); conn.close()
+    # Default to true to avoid breaking existing tenants that sell at $0.
+    allow = True
+    if row:
+        allow = str(row[0]).lower() in ('true', '1', 'yes')
+    return jsonify({'allow_zero_price_sales': allow})
+
+
+@config_bp.route('/sales-settings', methods=['PUT'])
+@require_auth('config.edit')
+def update_sales_settings():
+    """Update sales-related settings."""
+    data = request.get_json() or {}
+    allow = 'true' if data.get('allow_zero_price_sales') else 'false'
+    conn = get_tenant_conn(g.tenant_id)
+    cur = conn.cursor()
+    cur.execute("""
+        INSERT INTO tenant_config (key, value) VALUES (%s, %s)
+        ON CONFLICT (key) DO UPDATE SET value = EXCLUDED.value
+    """, ('allow_zero_price_sales', allow))
+    conn.commit()
+    cur.close(); conn.close()
+    return jsonify({'allow_zero_price_sales': allow == 'true'})
+
+
 @config_bp.route('/onboarding-status', methods=['GET'])
 @require_auth('pos.view')
 def get_onboarding_status():