feat(demo): Mail-Benachrichtigungen bei Demo-Ablauf

Neue Mail-Templates (lib/mail.js):
- sendDemoExpiryReminderMail: 7 Tage vor Ablauf
- sendDemoLastDayMail: 1 Tag vor Ablauf (Letzter-Tag-Reminder)
- sendDemoExpiredMail: am Ablauftag (setzt Tenant-Status abgelaufen)

Cron-Job (lib/expiryJob.js):
- node-cron taeglich 08:00 UTC (anpassbar via CRON_EXPIRY_AT)
- 3 Windows pruefen, idempotent ueber notified_*_at Timestamp-Spalten
- BETWEEN-Fenster [n-1, n] Tage fuer Robustheit

DB-Migration (bootstrap.js):
- Beim Backend-Start automatisches ALTER TABLE fuer
  notified_7d_at, notified_1d_at, notified_expired_at (idempotent)

Manueller Test: docker exec pfandsystem-demo-backend node -e "
  import(\"./lib/expiryJob.js\").then(m => m.runExpiryCheck())"

backend/lib/expiryJob.js

@@ -0,0 +1,93 @@
+import cron from 'node-cron';
+import pool from '../config/database.js';
+import {
+  sendDemoExpiryReminderMail,
+  sendDemoLastDayMail,
+  sendDemoExpiredMail,
+} from './mail.js';
+
+// Findet Demo-Tenants, deren Ablauf in [from, to] liegt UND fuer die noch
+// keine entsprechende Benachrichtigung versandt wurde, schickt die Mail
+// und markiert die Spalte mit NOW().
+async function processWindow({ flagColumn, daysFrom, daysTo, sender }) {
+  const [rows] = await pool.query(
+    `SELECT id, name, kontakt_email, slug, demo_expires_at
+       FROM tenants
+      WHERE typ = 'demo'
+        AND kontakt_email IS NOT NULL
+        AND ${flagColumn} IS NULL
+        AND demo_expires_at IS NOT NULL
+        AND demo_expires_at BETWEEN DATE_ADD(NOW(), INTERVAL ? DAY)
+                                AND DATE_ADD(NOW(), INTERVAL ? DAY)`,
+    [daysFrom, daysTo]
+  );
+  for (const t of rows) {
+    try {
+      await sender(t);
+      await pool.query(`UPDATE tenants SET ${flagColumn} = NOW() WHERE id = ?`, [t.id]);
+      console.log(`[expiry] ${flagColumn} -> ${t.kontakt_email} (slug=${t.slug})`);
+    } catch (e) {
+      console.error(`[expiry] Mail-Fehler ${flagColumn} fuer ${t.kontakt_email}:`, e.message);
+    }
+  }
+}
+
+async function processExpiredToday() {
+  const [rows] = await pool.query(
+    `SELECT id, name, kontakt_email, slug, demo_expires_at
+       FROM tenants
+      WHERE typ = 'demo'
+        AND kontakt_email IS NOT NULL
+        AND notified_expired_at IS NULL
+        AND demo_expires_at IS NOT NULL
+        AND demo_expires_at <= NOW()`
+  );
+  for (const t of rows) {
+    try {
+      await sendDemoExpiredMail({ to: t.kontakt_email, name: t.name });
+      await pool.query(
+        `UPDATE tenants SET notified_expired_at = NOW(),
+                            status = 'abgelaufen'
+         WHERE id = ?`,
+        [t.id]
+      );
+      console.log(`[expiry] expired-mail -> ${t.kontakt_email} (slug=${t.slug})`);
+    } catch (e) {
+      console.error(`[expiry] expired-Mail-Fehler ${t.kontakt_email}:`, e.message);
+    }
+  }
+}
+
+export async function runExpiryCheck() {
+  console.log('[expiry] check start', new Date().toISOString());
+  // 7-Tage-Erinnerung: Ablauf in [6, 7] Tagen
+  await processWindow({
+    flagColumn: 'notified_7d_at',
+    daysFrom: 6, daysTo: 7,
+    sender: (t) => sendDemoExpiryReminderMail({
+      to: t.kontakt_email, name: t.name, slug: t.slug,
+      daysLeft: 7, expiresAt: t.demo_expires_at,
+    }),
+  });
+  // 1-Tag-Erinnerung: Ablauf in [0, 1] Tagen (aber noch nicht abgelaufen)
+  await processWindow({
+    flagColumn: 'notified_1d_at',
+    daysFrom: 0, daysTo: 1,
+    sender: (t) => sendDemoLastDayMail({
+      to: t.kontakt_email, name: t.name, slug: t.slug,
+      expiresAt: t.demo_expires_at,
+    }),
+  });
+  // Heute (oder bereits) abgelaufen
+  await processExpiredToday();
+  console.log('[expiry] check done');
+}
+
+export function startExpiryCron() {
+  // Taeglich um 08:00 Uhr (Server-Zeit = UTC). Anpassbar via env CRON_EXPIRY_AT
+  const expr = process.env.CRON_EXPIRY_AT || '0 8 * * *';
+  cron.schedule(expr, () => {
+    runExpiryCheck().catch(e => console.error('[expiry] cron error:', e));
+  });
+  console.log('[expiry] cron registriert:', expr);
+}