Adds idempotency check for existing OOO calendar events

[tamaradement]

PR Description

• Adds idempotency check for existing OOO calendar events before inserting new ones.

Test Steps

• Add CLOCKIFY_FORCE_USER_ID to your .env file and set it to your own Clockify user ID.
• Run the program with parameters that intentionally reprocess an existing request.
• Verify that the existing OOO event is detected and printed in the logs and no duplicate OOO event is created in google calendar.