When the tracker restarts, nodes' registrations are lost. The heartbeat loop
was catching the 404 and printing a warning but never re-registering, leaving
the node permanently invisible until manually restarted.
Fix: on HTTP 404 heartbeat response, the loop re-posts the original
registration payload to /v1/nodes/register and updates the node_id and
heartbeat URL for subsequent beats. This also handles tracker expiry races.
The register_payload is now passed into _start_heartbeat so the thread has
everything needed for re-registration without reaching back into run_startup.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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