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.ts

@@ -101,6 +101,7 @@ export function parseCwPreviewConfig(raw: string): PreviewConfig {
       ...(svc.healthcheck !== undefined && { healthcheck: svc.healthcheck as PreviewServiceConfig['healthcheck'] }),
       ...(svc.env !== undefined && { env: svc.env as Record<string, string> }),
       ...(svc.volumes !== undefined && { volumes: svc.volumes as string[] }),
+      ...(Array.isArray(svc.seed) && { seed: svc.seed as string[] }),
       ...(svc.dev !== undefined && {
         dev: {
           image: (svc.dev as Record<string, unknown>).image as string,