fix: return JSON body from send-otp endpoint

Flutter client calls jsonDecode on response body — empty void response
throws FormatException even when SMS was sent successfully.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/auth/auth.controller.ts

@@ -55,8 +55,9 @@ export class AuthController {
   }
 
   @Post('send-otp')
-  sendOtp(@Body() dto: SendOtpDto) {
+  async sendOtp(@Body() dto: SendOtpDto) {
-    return this.auth.sendPhoneOtp(dto.phone);
+    await this.auth.sendPhoneOtp(dto.phone);
+    return { ok: true };
   }
 
   @Post('phone')