feat: publish maintenance beacon when ATM is under service

In maintenance mode, establish a minimal Nostr connection (no
Lightning.Pub) and publish Kind 30078 heartbeat with
maintenance: true. Monitors show yellow dot + "under service"
instead of appearing offline.

Only the Nostr client is initialized — no payment infrastructure.
Same one-shot private key security model as normal operation.

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

apps/machine/src/App.vue

@@ -43,6 +43,40 @@ onMounted(async () => {
 
   if (maintenanceMode) {
     atmStore.initError = 'maintenance'
+    // Publish maintenance beacon — minimal Nostr connection only (no Lightning.Pub)
+    try {
+      const { NostrClient, loadIdentityFromHex, createSignedEvent } = await import(
+        '@lamassu/nostr-client'
+      )
+      const secrets = isElectron ? await window.electronAPI?.getAtmSecrets() : null
+      const privKey = secrets?.atmPrivateKey || import.meta.env.VITE_ATM_PRIVATE_KEY
+      const relayUrl = config?.relayUrl || import.meta.env.VITE_RELAY_URL
+      if (privKey && relayUrl) {
+        const identity = loadIdentityFromHex(privKey)
+        const client = new NostrClient({ relays: [{ url: relayUrl }], identity })
+        await client.connect()
+        const publishBeacon = () => {
+          const event = createSignedEvent(identity, {
+            kind: 30078,
+            created_at: Math.floor(Date.now() / 1000),
+            tags: [['d', 'atm-availability']],
+            content: JSON.stringify({
+              cash_in: false,
+              cash_out: false,
+              cash_level: 'none',
+              maintenance: true,
+              fiat: config?.fiatCode || 'USD',
+              model: config?.machineModel || 'atm',
+            }),
+          })
+          client.publish(event).catch(() => {})
+        }
+        publishBeacon()
+        setInterval(publishBeacon, 5 * 60 * 1000)
+      }
+    } catch (e) {
+      console.warn('[App] Failed to start maintenance beacon:', e)
+    }
     return
   }