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/docker-client.ts

@@ -120,6 +120,23 @@ export async function composeDown(projectName: string): Promise<void> {
   });
 }
 
+/**
+ * Execute a command inside a running service container.
+ * Used for seed commands (migrations, fixture loading, etc.) after health checks pass.
+ */
+export async function execInContainer(
+  projectName: string,
+  serviceName: string,
+  command: string,
+): Promise<{ stdout: string; stderr: string }> {
+  const result = await execa('docker', [
+    'compose', '-p', projectName,
+    'exec', '-T', serviceName,
+    'sh', '-c', command,
+  ], { timeout: 300000 }); // 5 min per seed command
+  return { stdout: result.stdout, stderr: result.stderr };
+}
+
 /**
  * Get service statuses for a compose project.
  */