Add calendar sync utilities and Beteiligte sync cron job

- Implemented calendar_sync_utils.py for shared utility functions
  including DB connection, Google Calendar service initialization,
  Redis client setup, and employee sync operations.

- Created beteiligte_sync_cron_step.py to handle periodic sync
  of Beteiligte entities, checking for new, modified, failed,
  and stale records, and emitting sync events accordingly.

pyproject.toml

@@ -14,5 +14,9 @@ dependencies = [
     "python-dotenv>=1.0.0",
     "pytz>=2025.2",
     "requests>=2.32.0",
+    "asyncpg>=0.29.0",  # PostgreSQL async driver for calendar sync
+    "google-api-python-client>=2.100.0",  # Google Calendar API
+    "google-auth>=2.23.0",  # Google OAuth2
+    "backoff>=2.2.1",  # Retry/backoff decorator
 ]