feat: implement Story 1.1 — create and view diagrams

Add diagram/project DB schema, CRUD API, dashboard pages with grid/card/
empty state, create dialog with type selector, editor placeholder, and
26 schema validation tests. Includes code review fixes: soft-delete
filter on GET /:id, error handling, keyboard accessibility, type-safe
API response types, and error states on pages.

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

packages/api/src/index.ts

@@ -13,6 +13,7 @@ import { adminRouter } from "./modules/admin/router";
 import { aiRouter } from "./modules/ai/router";
 import { authRouter } from "./modules/auth/router";
 import { billingRouter } from "./modules/billing/router";
+import { diagramRouter } from "./modules/diagram/router";
 import { organizationRouter } from "./modules/organization/router";
 import { storageRouter } from "./modules/storage/router";
 import { onError } from "./utils/on-error";
@@ -48,6 +49,7 @@ const appRouter = new Hono()
   .route("/ai", aiRouter)
   .route("/auth", authRouter)
   .route("/billing", billingRouter)
+  .route("/diagrams", diagramRouter)
   .route("/organizations", organizationRouter)
   .route("/storage", storageRouter)
   .onError(onError);