Add worker heartbeat migration

scripts/migrate-worker-heartbeat-db.sh

@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+DB_FILE="/opt/appfactory/data/appfactory/appfactory.db"
+
+python3 - "$DB_FILE" <<'PY'
+import sqlite3
+import sys
+
+db_file = sys.argv[1]
+con = sqlite3.connect(db_file)
+
+con.execute("""
+CREATE TABLE IF NOT EXISTS workers (
+    id TEXT PRIMARY KEY,
+    status TEXT NOT NULL DEFAULT 'online',
+    started_at TEXT NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    last_seen_at TEXT NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    current_job_id INTEGER,
+    metadata_json TEXT
+)
+""")
+
+con.execute("""
+CREATE INDEX IF NOT EXISTS idx_workers_last_seen
+ON workers(last_seen_at)
+""")
+
+con.commit()
+con.close()
+
+print("Worker heartbeat migration completed.")
+PY