Google Calendar OAuth2 Integration + CalDAV-Kalender ausblenden statt löschen

- Google OAuth2 Flow: Admin konfiguriert Client-ID/Secret, User verbindet per Klick
- Google Calendar API v3: Events lesen, erstellen, bearbeiten, löschen
- GoogleAccount Model + google_router mit Token-Refresh
- Google-Events in Event-Pipeline integriert
- Frontend: Google Kalender in Sidebar, Dropdown, Event-CRUD-Routing
- CalDAV-Kalender: Ausblenden statt ganzes Konto löschen, Einblenden in Einstellungen
- Ausgeblendete Kalender Sektion in Einstellungen

backend/main.py

@@ -12,7 +12,7 @@ from sqlalchemy import text
 sys.path.insert(0, str(Path(__file__).parent))
 
 from database import Base, engine
-from routers import auth_router, caldav_router, ical_router, local_router, profile_router, settings_router, users_router
+from routers import auth_router, caldav_router, google_router, ical_router, local_router, profile_router, settings_router, users_router
 
 logging.basicConfig(level=logging.INFO)
 
@@ -43,6 +43,7 @@ app.include_router(settings_router.router, prefix="/api/settings", tags=["settin
 app.include_router(profile_router.router, prefix="/api/profile", tags=["profile"])
 app.include_router(local_router.router, prefix="/api/local", tags=["local"])
 app.include_router(ical_router.router, prefix="/api/ical", tags=["ical"])
+app.include_router(google_router.router, prefix="/api/google", tags=["google"])
 
 FRONTEND_DIR = Path(__file__).parent.parent / "frontend"
 app.mount("/static", StaticFiles(directory=str(FRONTEND_DIR)), name="static")