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/prisma/schema.prisma

@@ -110,3 +110,27 @@ model TenantModuleActivation {
   @@unique([tenantId, moduleId])
   @@index([tenantId])
 }
+
+model DashboardLayout {
+  id        String   @id @default(uuid())
+  userId    String   @unique
+  tenantId  String
+  layouts   Json     @default("{}")
+  createdAt DateTime @default(now())
+  updatedAt DateTime @updatedAt
+
+  @@index([tenantId])
+}
+
+model WidgetInstance {
+  id         String   @id @default(uuid())
+  userId     String
+  tenantId   String
+  widgetType String
+  config     Json     @default("{}")
+  createdAt  DateTime @default(now())
+  updatedAt  DateTime @updatedAt
+
+  @@index([userId])
+  @@index([tenantId])
+}