feat(05-01): dashboard backend — Prisma models, CRUD API, module wiring

- Add DashboardLayout and WidgetInstance Prisma models with userId/tenantId scoping
- Create DashboardController with 6 endpoints (layout CRUD + widget CRUD)
- Create DashboardService with ownership verification on all widget mutations (T-05-01)
- Add SaveLayoutDto, CreateWidgetDto, UpdateWidgetConfigDto with class-validator
- Register DashboardModule in app.module.ts imports

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

apps/api/src/dashboard/dto/save-layout.dto.ts

@@ -0,0 +1,11 @@
+import { IsObject } from 'class-validator';
+
+/**
+ * DTO for saving/updating a user's dashboard layout.
+ * The layouts object contains responsive breakpoint layouts
+ * (lg, md, sm, xs, xxs) as managed by react-grid-layout.
+ */
+export class SaveLayoutDto {
+  @IsObject()
+  layouts!: Record<string, unknown>;
+}