Add customer Excel import/export and Gama bulk SMS sending.

Business SMS uses optional settings.sms.senderNumber (else advertising shortcode) with SendQuick for one recipient and SendBulk for quantity.

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

src/customers/dto/send-customer-sms.dto.ts

@@ -0,0 +1,8 @@
+import { IsString, MaxLength, MinLength } from 'class-validator';
+
+export class SendCustomerSmsDto {
+  @IsString()
+  @MinLength(1)
+  @MaxLength(700)
+  message!: string;
+}