Make the stuck-transaction monitor's thresholds node-configurable

[mswilkison]

The transactionMonitor added in #4174 uses compile-time constants for all of its tunables:

• defaultStuckTransactionThreshold (6h) — how long unconfirmed before "stuck"
• transactionMonitorCheckInterval (5m) — poll cadence
• transactionMonitorMaxTracked (1000) — tracking-table bound
• transactionMonitorMaxTrackingAge (24h) — give-up age
• transactionMonitorCheckBudget (2m) — per-pass time budget

None can be tuned without a code change and rebuild.

Follow-up (raised in review of #4174): move these into the tbtc node config with the current values as defaults, so operators/deployments can adjust the stuck threshold and cadence without recompiling.

Refs #4174.

[linear]

ENG-674