Fix: UTC fuer interne Timer, Berlin nur fuer Anzeige

Korrektur: Alle DB-Timestamps (refresh_log, created_at, updated_at,
auth, notifications) bleiben UTC fuer korrekte Timer-Vergleiche.
Europe/Berlin nur fuer angezeigte Werte (Exporte, Prompts, API).
Verhindert zu fruehes Ausloesen des Auto-Refresh-Timers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/auth.py

@@ -1,7 +1,7 @@
 """JWT-Authentifizierung mit Magic-Link-Support und Multi-Tenancy."""
 import secrets
 import string
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, timezone
 from jose import jwt, JWTError
 from fastapi import Depends, HTTPException, status
 from fastapi.security import HTTPBearer, HTTPAuthorizationCredentials
@@ -23,7 +23,7 @@ def create_token(
     org_slug: str = None,
 ) -> str:
     """JWT-Token erstellen mit Tenant-Kontext."""
-    now = datetime.now(TIMEZONE)
+    now = datetime.now(timezone.utc)
     expire = now + timedelta(hours=JWT_EXPIRE_HOURS)
     payload = {
         "sub": str(user_id),