Hardcode agent stale timeout

app/database.py

@@ -105,7 +105,6 @@ CREATE TABLE IF NOT EXISTS agent_activity (
 _DEFAULT_SETTINGS = {
     "default_wait_seconds": "10",
     "default_empty_response": DEFAULT_EMPTY_RESPONSE,
-    "agent_stale_after_seconds": "30",
 }
 
 
@@ -125,6 +124,7 @@ def _seed_defaults(conn: sqlite3.Connection) -> None:
         "UPDATE settings SET value = ? WHERE key = 'default_empty_response' AND value = ''",
         (_DEFAULT_SETTINGS["default_empty_response"],),
     )
+    conn.execute("DELETE FROM settings WHERE key = 'agent_stale_after_seconds'")
     conn.commit()
     logger.debug("Default settings seeded")