feat(orders): credit each order to the expert who placed it, as legacy did

Legacy fixed `orders.reagent_id` once, when the order was placed, and its
«میزان فروش» summed `customer_id = X OR reagent_id = X`. The app credited
orders to the customer's CURRENT referrer and left out an expert's own
orders, so 9 of 49 imported experts showed a different figure.

- Order.salesAgentId (+ referrerCustomerId for 17 legacy orders whose
  referrer was not an expert); additive migration, recent app orders
  backfilled from the customer's referrer
- createOrder records the expert at placement time
- one shared filter (agent-orders.ts) for the experts list, expert
  detail, dashboard and the expert app; child orders follow salesAgentId
- customer page gets its «میزان فروش» back
- import carries orders.reagent_id as is; ex-experts kept as admins keep
  their profile so their own orders stay linked
- reconcile-sales: 1,557 of 1,558 match legacy's own formula; balances
  unchanged

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/api/prisma/schema.prisma

@@ -179,6 +179,8 @@ model AdminUser {
   memberCode   String? // legacy mcode
 
   referredCustomers    Customer[]            @relation("ReferralAgent")
+  /// Orders credited to this sales expert — see Order.salesAgentId.
+  creditedOrders       Order[]               @relation("OrderSalesAgent")
   supervisedWorkshops  Workshop[]            @relation("WorkshopSupervisor")
   branchAllocations    BranchAllocation[]    @relation("AllocationAuthor")
   postedLedgerEntries  LedgerEntry[]
@@ -421,6 +423,9 @@ model Customer {
   /// account (see SalesExpertProfile.tradingCustomerId). Empty for every
   /// ordinary customer.
   salesExpertTradingFor SalesExpertProfile? @relation("SalesExpertTradingAccount")
+  /// Legacy orders that named THIS customer as their referrer — see
+  /// Order.referrerCustomerId. History only.
+  referredOrders        Order[]             @relation("OrderReferrerCustomer")
 
   // Credit limit + cached ledger balances (source of truth is LedgerEntry; these
   // are denormalized for fast reads and reconciled by a scheduled job).
@@ -736,6 +741,22 @@ model Order {
   isSuspendedOverCreditLimit Boolean   @default(false)
   deliveredAt                DateTime? // setting this is what triggers debt recognition
 
+  /// The sales expert this order is credited to, fixed when it is placed:
+  /// the customer's referring expert AT THAT MOMENT. Legacy `orders.reagent_id`,
+  /// set once at creation (ApiMobileController.php:601) and never moved, so a
+  /// customer changing expert later does not move past sales or commission
+  /// (owner ruling 2026-10-08). «میزان فروش» counts from here, never from the
+  /// customer's current referrer.
+  salesAgentId String?
+  salesAgent   AdminUser? @relation("OrderSalesAgent", fields: [salesAgentId], references: [id], onDelete: SetNull)
+
+  /// History only: a legacy order whose `reagent_id` named a customer who is
+  /// not a sales expert (one such customer, 17 orders in 2022). Legacy counted
+  /// those orders in that customer's own «میزان فروش»; this keeps that figure
+  /// exact. Never set for a new order.
+  referrerCustomerId String?
+  referrerCustomer   Customer? @relation("OrderReferrerCustomer", fields: [referrerCustomerId], references: [id], onDelete: SetNull)
+
   createdAt DateTime @default(now())
   updatedAt DateTime @updatedAt
 
@@ -746,6 +767,8 @@ model Order {
 
   @@index([customerId])
   @@index([status])
+  @@index([salesAgentId])
+  @@index([referrerCustomerId])
 }
 
 model OrderItem {