feat: add cash_level to availability broadcast

Publish cash level (none/low/good/full) in the Kind 30078 event
based on total bill count across all cassettes. Enables monitoring
dashboards to show cash availability without revealing exact amounts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/machine/src/composables/useAvailabilityBroadcast.ts

@@ -48,10 +48,20 @@ export function useAvailabilityBroadcast(options: UseAvailabilityBroadcastOption
     return { cashIn, cashOut }
   }
 
+  /** Cash level: none / low / good / full based on total bills */
+  function cashLevel(): string {
+    const totalBills = Object.values(inventory.value).reduce((s, c) => s + c, 0)
+    if (totalBills === 0) return 'none'
+    if (totalBills < 20) return 'low'
+    if (totalBills < 80) return 'good'
+    return 'full'
+  }
+
   async function publish(avail: ATMAvailability) {
     const content = JSON.stringify({
       cash_in: avail.cashIn,
       cash_out: avail.cashOut,
+      cash_level: cashLevel(),
       fiat: fiatCode,
       model,
     })