fix: UTF-8 encoding for iCal subscriptions + default_event_duration_minutes setting

Force resp.encoding='utf-8' in _fetch_ics so umlauts (ö/ä/ü) from sources
like Ferienwiki.de are decoded correctly instead of showing as ö/ä/ü.

Add default_event_duration_minutes to UserSettings (nullable int, same pattern
as default_reminder_minutes) with DB migration, GET/PUT in settings_router.

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

backend/routers/ical_router.py

@@ -50,6 +50,7 @@ def _fetch_ics(url: str) -> str:
     try:
         resp = http_requests.get(url, timeout=30, allow_redirects=True)
         resp.raise_for_status()
+        resp.encoding = 'utf-8'
         return resp.text
     except http_requests.RequestException as e:
         raise ValueError(f"Fehler beim Abrufen der URL: {e}")