fix(api): prevent self-loop routes, map P2003 to 400, forward DELETE error body

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

apps/api/src/modules/routes/routes.service.ts

@@ -23,14 +23,22 @@ export class RoutesService {
     if (!sourceGroupId || !targetGroupId) {
       throw new BadRequestException('sourceGroupId and targetGroupId are required');
     }
+    if (sourceGroupId === targetGroupId) {
+      throw new BadRequestException('Source and target groups cannot be the same');
+    }
     try {
       return await this.prisma.syncRoute.create({
         data: { sourceGroupId, targetGroupId },
         include: routeInclude,
       });
     } catch (e) {
-      if (e instanceof Prisma.PrismaClientKnownRequestError && e.code === 'P2002') {
-        throw new ConflictException('A route between these groups already exists');
+      if (e instanceof Prisma.PrismaClientKnownRequestError) {
+        if (e.code === 'P2002') {
+          throw new ConflictException('A route between these groups already exists');
+        }
+        if (e.code === 'P2003') {
+          throw new BadRequestException('One or both group IDs do not exist');
+        }
       }
       throw e;
     }