fix(sync): dedup order_items by sku before insert to avoid UNIQUE crash

Production sync was failing every minute with:
  UNIQUE constraint failed: order_items.order_number, order_items.sku

GoMag occasionally returns the same SKU on multiple lines within one order
(configurable products, promo splits). The order_items PK is
(order_number, sku), so the raw batch insert violates UNIQUE and aborts
the entire sync — blocking partner-mismatch updates, address refresh,
and items repopulation for already-imported orders.

Added _dedup_items_by_sku() helper. Applied in save_orders_batch
(cancelled/already/skipped paths) and add_order_items (retry/sync import
paths). Keeps first price/vat/name, sums quantities on collision.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

api/app/services/sqlite_service.py

@@ -193,12 +193,16 @@ async def save_orders_batch(orders_data: list[dict]):
             VALUES (?, ?, ?)
         """, [(d["sync_run_id"], d["order_number"], d["status_at_run"]) for d in orders_data])
 
-        # 3. Order items — replace semantics (GoMag source of truth)
+        # 3. Order items — replace semantics (GoMag source of truth).
+        # Dedup per-order by SKU (GoMag sometimes returns same SKU twice).
         all_items = []
         order_numbers_with_items = set()
         for d in orders_data:
-            for item in d.get("items", []):
-                order_numbers_with_items.add(d["order_number"])
+            raw_items = d.get("items", [])
+            if not raw_items:
+                continue
+            order_numbers_with_items.add(d["order_number"])
+            for item in _dedup_items_by_sku(raw_items):
                 all_items.append((
                     d["order_number"],
                     item.get("sku"), item.get("product_name"),
@@ -535,14 +539,40 @@ async def get_web_products_batch(skus: list) -> dict:
 
 # ── order_items ──────────────────────────────────
 
+def _dedup_items_by_sku(items: list) -> list:
+    """Deduplicate items by SKU within a single order. Sums quantities on collision.
+    GoMag occasionally returns the same SKU on multiple lines (configurable products,
+    promo splits). The order_items primary key is (order_number, sku) so the raw rows
+    would violate UNIQUE. Keeps first price/vat/name; sums quantity + baseprice*qty.
+    """
+    if not items:
+        return items
+    merged: dict = {}
+    order: list = []
+    for item in items:
+        sku = item.get("sku")
+        if sku is None:
+            order.append(item)
+            continue
+        if sku in merged:
+            prev = merged[sku]
+            prev["quantity"] = (prev.get("quantity") or 0) + (item.get("quantity") or 0)
+        else:
+            merged[sku] = dict(item)
+            order.append(merged[sku])
+    return order
+
+
 async def add_order_items(order_number: str, items: list):
     """Replace order items — delete any existing rows, then insert fresh batch.
 
     GoMag is source of truth: re-import must reflect quantity changes.
-    Atomic (DELETE + INSERT in one transaction).
+    Atomic (DELETE + INSERT in one transaction). Items with the same SKU are
+    merged (quantities summed) to satisfy the (order_number, sku) PK.
     """
     if not items:
         return
+    items = _dedup_items_by_sku(items)
     db = await get_sqlite()
     try:
         await db.execute("DELETE FROM order_items WHERE order_number = ?", (order_number,))