feat: Add seed command support to preview deployments

Run project-specific initialization commands (DB migrations, fixture
loading, etc.) automatically after containers are healthy, before the
preview is marked ready. Configured via per-service `seed` arrays in
.cw-preview.yml.

apps/server/preview/config-reader.test.ts

@@ -133,6 +133,33 @@ services:
     expect(config.services.frontend.dev!.workdir).toBe('/app');
   });
 
+  it('parses seed array on a service', () => {
+    const raw = `
+version: 1
+services:
+  app:
+    build: "."
+    port: 3000
+    seed:
+      - npm run db:migrate
+      - npm run db:seed
+`;
+    const config = parseCwPreviewConfig(raw);
+    expect(config.services.app.seed).toEqual(['npm run db:migrate', 'npm run db:seed']);
+  });
+
+  it('omits seed when not specified', () => {
+    const raw = `
+version: 1
+services:
+  app:
+    build: "."
+    port: 3000
+`;
+    const config = parseCwPreviewConfig(raw);
+    expect(config.services.app.seed).toBeUndefined();
+  });
+
   it('parses dev section with only image', () => {
     const raw = `
 version: 1