Robot Heartbeat endpoint

POST /robot/heartbeat. Authorization: Bearer JWT. Body: sequence (incrementing), timestamp (ISO 8601), dynamic_data (balance, equity, margin, drawdown, P/L). Robots add config_change_results and symbols_change_results when pending.

If token near expiry, call /robot/refresh first. Build payload, POST /robot/heartbeat. On 200: increment sequence, update last heartbeat (TimeLocal()), process interval/termination/change requests (Robots). On 409: sync sequence from context.expected_sequence or context.current_sequence, retry next interval.