feat(api): add GroupsModule with GET /groups endpoint

Implements TDD-driven GroupsService and GroupsController with full unit test coverage. GET /groups returns all groups ordered by name, selecting id, name, platform, platformId, isActive, and accountId fields.

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

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

@@ -0,0 +1,14 @@
+import { Injectable } from '@nestjs/common';
+import { PrismaService } from '../../prisma/prisma.service';
+
+@Injectable()
+export class GroupsService {
+  constructor(private readonly prisma: PrismaService) {}
+
+  list() {
+    return this.prisma.group.findMany({
+      orderBy: { name: 'asc' },
+      select: { id: true, name: true, platform: true, platformId: true, isActive: true, accountId: true },
+    });
+  }
+}