Send heartbeat ping (dead man's switch)

Send a heartbeat ping to keep your orders alive. If no heartbeat is received within the configured timeout period, all open orders for the maker are automatically cancelled.

This acts as a dead man's switch for automated trading systems: if your bot crashes or loses connectivity, your orders are cancelled to prevent stale orders sitting on the book.

How it works:

1. Call POST /heartbeat with your desired timeout (in seconds)
2. The server records the heartbeat configuration and last ping time
3. Continue calling periodically (before the timeout expires)
4. If a ping is missed beyond the timeout, all your open orders are cancelled

Authentication Options:

• Signature: Include signature and signature_deadline fields (uses EIP-712 HeartbeatType)
• One-click session: Include x-one-click header with session hash (no signature needed)

Replay Protection: When using signatures, the signature_deadline must be strictly greater than the last accepted deadline for this maker (deadline monotonicity). This prevents replay attacks without requiring per-call nonce storage. Deadlines are also bounded to at most 30 seconds in the future.