Add email notifications, SMTP settings and customer archive lifecycle

- Optional email field on users and customers
- SMTP config in admin settings with test-mail button and an opt-in
  "notify admins on upload" toggle
- Debounced upload notifier sends one summary email per customer session
  to the customer, assigned staff and (optionally) admins
- Two-step customer lifecycle: "Deaktivieren" archives the link and
  keeps data, "Dateien löschen" purges files and the DB entry after a
  name-typed confirmation

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

src/db.js

@@ -62,4 +62,15 @@ db.exec(`
   );
 `);
 
+// Additive migrations for existing DBs
+function addColumnIfMissing(table, column, ddl) {
+  const cols = db.prepare(`PRAGMA table_info(${table})`).all();
+  if (!cols.some(c => c.name === column)) {
+    db.exec(`ALTER TABLE ${table} ADD COLUMN ${column} ${ddl}`);
+  }
+}
+addColumnIfMissing('users', 'email', 'TEXT');
+addColumnIfMissing('customers', 'email', 'TEXT');
+addColumnIfMissing('customers', 'archived_at', 'INTEGER');
+
 module.exports = db;