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/create-widget.dto.ts

@@ -0,0 +1,16 @@
+import { IsIn, IsObject, IsOptional, IsString } from 'class-validator';
+
+/**
+ * DTO for creating a new widget instance on a user's dashboard.
+ * widgetType must be one of the four supported types.
+ * config is optional and defaults to {} on the model.
+ */
+export class CreateWidgetDto {
+  @IsString()
+  @IsIn(['clock', 'search', 'calendar', 'note'])
+  widgetType!: string;
+
+  @IsOptional()
+  @IsObject()
+  config?: Record<string, unknown>;
+}