FLOWIP-063e: Runtime heartbeats and liveness signals

[rocketpages]

Checklist

• I read CONTRIBUTING.md
• All commits are signed off (DCO): git commit -s
• I ran: cargo fmt --all
• I ran: cargo test --workspace

Summary

• Add a background heartbeat task per stage with shared in-memory liveness state
• Emit liveness state changes for links between stages to the system event log (only on change)
• Feed liveness into the UI event stream (SSE) and export a Prometheus gauge for alerting
• Make liveness signals non-fatal (they never trigger a pipeline abort)
• Reduce stall-noise (debounce repeats, raise defaults) and add a deterministic “slow but healthy” regression test