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>

package.json

@@ -12,6 +12,7 @@
     "express": "^4.21.0",
     "express-basic-auth": "^1.2.1",
     "multer": "^1.4.5-lts.1",
-    "nanoid": "^3.3.7"
+    "nanoid": "^3.3.7",
+    "nodemailer": "^6.9.15"
   }
 }