mcp: throttle async job poll requests

crates/crank-registry/src/migrations.rs

@@ -558,6 +558,7 @@ pub async fn apply_postgres(pool: &PgPool) -> Result<(), sqlx::Error> {
             result_json jsonb null,
             error_json jsonb null,
             expires_at timestamptz null,
+            last_poll_at timestamptz null,
             created_at timestamptz not null,
             updated_at timestamptz not null,
             finished_at timestamptz null
@@ -566,6 +567,10 @@ pub async fn apply_postgres(pool: &PgPool) -> Result<(), sqlx::Error> {
     .execute(pool)
     .await?;
 
+    query("alter table async_jobs add column if not exists last_poll_at timestamptz null")
+        .execute(pool)
+        .await?;
+
     query(
         "create index if not exists async_jobs_workspace_status_idx
          on async_jobs(workspace_id, status, updated_at desc)",