fix: creating a board without a description returns 500

`description` is optional in the request schema, but the column is
NOT NULL with no default, so an omitted description reached Prisma as
undefined and the create threw a validation error instead of saving:

    Invalid `prisma.board.create()` invocation
    Argument `description` is missing.

Default it to an empty string, matching what the UI sends today, and
cover it with an API test.

src/app/api/boards/route.ts

@@ -75,6 +75,9 @@ export async function POST(request: Request) {
     ...body,
     type: normalizeBoardType(body.type),
     id: uuid(),
+    // The column is NOT NULL with no default, so an omitted description —
+    // which the request schema allows — reaches Prisma as undefined and throws.
+    description: body.description ?? '',
     parameters: body.parameters ?? {},
     userId: !teamId ? auth.user.id : undefined,
   };

tests/e2e/api-board.spec.ts

@@ -0,0 +1,35 @@
+import { expect, test } from '@playwright/test';
+import { type Auth, authHeaders, loginViaApi } from './helpers';
+
+test.describe('Board API tests', () => {
+  test.describe.configure({ mode: 'serial' });
+
+  let auth: Auth;
+  let boardId = '';
+
+  test.beforeAll(async ({ request }) => {
+    auth = await loginViaApi(request);
+  });
+
+  test.afterAll(async ({ request }) => {
+    if (boardId) {
+      await request.delete(`/api/boards/${boardId}`, { headers: authHeaders(auth) });
+    }
+  });
+
+  test('creates a board without a description', async ({ request }) => {
+    const response = await request.post('/api/boards', {
+      headers: authHeaders(auth),
+      data: { name: 'Playwright board', type: 'mixed' },
+    });
+
+    expect(response.status()).toBe(200);
+
+    const body = await response.json();
+
+    boardId = body.id;
+
+    expect(body).toHaveProperty('name', 'Playwright board');
+    expect(body).toHaveProperty('description', '');
+  });
+});