refactor: remove module-level sync_user() and get_valid_token()

[sebastiaankok]

Parent

#51

What to build

Delete the module-level sync_user() and get_valid_token() functions from the calendar sync module. Verify no remaining references exist anywhere in the codebase. Remove any test cases that tested these functions directly (all behaviour is now covered by CalendarSyncService tests).

Acceptance criteria

• Module-level sync_user() deleted
• Module-level get_valid_token() deleted
• No import or call of calendar_sync.sync_user or calendar_sync.get_valid_token anywhere in the codebase
• No test references the removed functions
• Full test suite green

Blocked by

• refactor: migrate all sync call sites to CalendarSyncService #53