feat: dual pricing policies + discount VAT splitting

Add production pricing policy (id_pol_productie) for articles with cont 341/345,
smart discount VAT splitting across multiple rates, per-article id_pol support,
and mapped SKU price validation. Settings UI updated with new controls.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/app/database.py

@@ -110,7 +110,8 @@ CREATE TABLE IF NOT EXISTS orders (
     order_total           REAL,
     delivery_cost         REAL,
     discount_total        REAL,
-    web_status          TEXT
+    web_status          TEXT,
+    discount_split      TEXT
 );
 CREATE INDEX IF NOT EXISTS idx_orders_status ON orders(status);
 CREATE INDEX IF NOT EXISTS idx_orders_date ON orders(order_date);
@@ -318,6 +319,7 @@ def init_sqlite():
             ("delivery_cost", "REAL"),
             ("discount_total", "REAL"),
             ("web_status", "TEXT"),
+            ("discount_split", "TEXT"),
         ]:
             if col not in order_cols:
                 conn.execute(f"ALTER TABLE orders ADD COLUMN {col} {typedef}")