feat: add utility function for verification link expiration check

- Introduced a new function to determine if a verification link has expired, improving error handling in the email verification process.
- Updated the email verification logic to utilize the new function, enhancing clarity and maintainability of the code.

backend/routers/auth.py

@@ -168,6 +168,18 @@ def _public_app_base() -> str:
     return (os.getenv("APP_URL") or "https://shinkan.jinkendo.de").rstrip("/")
 
 
+def _verification_link_expired(expires_at) -> bool:
+    """
+    True, wenn Ablaufzeit in der Vergangenheit liegt.
+    PG liefert TIMESTAMP oft als naive datetime — Vergleich mit timezone-aware UTC
+    würde sonst TypeError → 500.
+    """
+    if expires_at is None:
+        return False
+    deadline = expires_at.replace(tzinfo=timezone.utc) if expires_at.tzinfo is None else expires_at.astimezone(timezone.utc)
+    return datetime.now(timezone.utc) > deadline
+
+
 def verification_link(token: str) -> str:
     """Link zur Web-App (`/verify?token=`); die SPA ruft wie bei Mitai die API auf."""
     return f"{_public_app_base()}/verify?token={quote(token, safe='')}"
@@ -340,7 +352,7 @@ async def verify_email(token: str):
             raise HTTPException(400, "E-Mail-Adresse bereits bestätigt")
 
         # Check if token expired
-        if prof['verification_expires'] and datetime.now(timezone.utc) > prof['verification_expires']:
+        if _verification_link_expired(prof["verification_expires"]):
             raise HTTPException(400, "Verifikations-Link abgelaufen. Bitte registriere dich erneut.")
 
         # Mark as verified and clear token