fix(presence): missing touchLastSeen import in /api/me/last-seen

Endpoint warf 500 jedem Heartbeat-Call (ReferenceError) — Floodete
pm2-Logs. Import war im DB-Layer-File implementiert aber nicht vom
Endpoint importiert.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/server/api/me/last-seen.post.ts

@@ -11,6 +11,7 @@
  * Response: { lastSeenAt: ISOString }
  */
 import { requireUser } from "../../utils/auth";
+import { touchLastSeen } from "../../db/profile";
 
 export default defineEventHandler(async (event) => {
   const user = await requireUser(event);