Validate user presence assumptions for Key model

Summary

The Key model delegates to user but doesn't enforce user presence via Rails validations or database constraints. This creates ambiguity about whether key.user can be nil.

Investigation Needed

1. Determine if user_id: nil is a valid state for keys
2. Check production data for keys with NULL user_id
3. Either add validation/constraints OR update code to handle nil users consistently

Context

During code review, it was discovered that while the Key model delegates to user (suggesting it should always be present), Rails and the database don't actually prevent user_id from being nil.