feat(customers): add optional group assignees with OR visibility for people, companies, and birthdays

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/api/prisma/schema.prisma

@@ -306,6 +306,7 @@ model User {
   ownedInvoices          Invoice[]                @relation("InvoiceOwner")
   ownedPeople            Person[]                 @relation("PersonOwner")
   ownedCompanies         Company[]                @relation("CompanyOwner")
+  assignedCustomerGroups CustomerGroup[]          @relation("CustomerGroupAssignees")
   financialPartyDocuments FinancialDocument[]     @relation("FinancialDocumentParty")
   ownedProjectCards      ProjectCard[]            @relation("ProjectCardOwner")
   activities             Activity[]               @relation("ActivityAssignee")
@@ -576,6 +577,8 @@ model CustomerGroup {
   children            CustomerGroup[]    @relation("CustomerGroupTree")
   people              PersonGroupLink[]
   companies           CompanyGroupLink[]
+  /// مسئولان گروه — اختیاری؛ اعضای گروه را مثل مشتریان خود می‌بینند
+  assignees           User[]             @relation("CustomerGroupAssignees")
   createdAt           DateTime           @default(now())
   updatedAt           DateTime           @updatedAt