AuthController: Return 409 Conflict when user already exists

The previously used HTTP error 400 'Bad Request' is not really
applicable here, as the client did not send a malformed message.

Signed-off-by: David Mehren <git@herrmehren.de>

src/api/private/auth/auth.controller.ts

@@ -6,6 +6,7 @@
 import {
   BadRequestException,
   Body,
+  ConflictException,
   Controller,
   Delete,
   NotFoundException,
@@ -56,7 +57,7 @@ export class AuthController {
       return;
     } catch (e) {
       if (e instanceof AlreadyInDBError) {
-        throw new BadRequestException(e.message);
+        throw new ConflictException(e.message);
       }
       throw e;
     }