feat: add item-level order confirmation/decline support

Per Letzshop API, each inventory unit must be confirmed/declined individually.
This enables partial confirmation (some items confirmed, others declined).

Admin API endpoints:
- POST /vendors/{id}/orders/{id}/confirm - confirm all items
- POST /vendors/{id}/orders/{id}/reject - decline all items
- POST /vendors/{id}/orders/{id}/items/{id}/confirm - confirm single item
- POST /vendors/{id}/orders/{id}/items/{id}/decline - decline single item

Order detail modal now shows:
- Product name, EAN, SKU, MPN, price per item
- Per-item state badge (unconfirmed/confirmed/declined)
- Per-item confirm/decline buttons for pending items
- Bulk confirm/decline all buttons

Order status logic:
- If all items declined -> order is "declined"
- If any item confirmed -> order is "confirmed"
- Partial confirmation supported

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/services/letzshop/order_service.py

@@ -403,6 +403,56 @@ class LetzshopOrderService:
         order.sync_status = "rejected"
         return order
 
+    def update_inventory_unit_state(
+        self, order: LetzshopOrder, item_id: str, state: str
+    ) -> LetzshopOrder:
+        """
+        Update the state of a single inventory unit in an order.
+
+        Args:
+            order: The order containing the inventory unit.
+            item_id: The inventory unit ID to update.
+            state: The new state (confirmed_available, confirmed_unavailable).
+
+        Returns:
+            The updated order.
+        """
+        if not order.inventory_units:
+            return order
+
+        # Update the specific item's state
+        updated_units = []
+        for unit in order.inventory_units:
+            if unit.get("id") == item_id:
+                unit["state"] = state
+            updated_units.append(unit)
+
+        order.inventory_units = updated_units
+
+        # Check if all items are now processed and update order status accordingly
+        all_confirmed = all(
+            u.get("state") in ("confirmed_available", "confirmed_unavailable", "returned")
+            for u in updated_units
+        )
+
+        if all_confirmed:
+            # Determine order status based on item states
+            has_available = any(
+                u.get("state") == "confirmed_available" for u in updated_units
+            )
+            all_unavailable = all(
+                u.get("state") == "confirmed_unavailable" for u in updated_units
+            )
+
+            if all_unavailable:
+                order.sync_status = "rejected"
+                order.rejected_at = datetime.now(UTC)
+            elif has_available:
+                order.sync_status = "confirmed"
+                order.confirmed_at = datetime.now(UTC)
+
+        return order
+
     def set_order_tracking(
         self,
         order: LetzshopOrder,