docs(users): explain deletion conflicts and cleanup ordering

[grevgeny]

Summary

Document the v4 user deletion response when a loaded SQL row disappears, and explain when identity and Mongo cleanup begin.

Refs ditioas/timestyr#5592.

Changes

Add a DELETE example and describe the 204, 404 and 409 results, readback before retry, and possible reconciliation after later cleanup failure.

Test plan

Documentation diff and whitespace checks passed. CI builds the examples.

Risk

Documentation only. This describes the coordinated Core API change; it does not change sample execution or delete data.

[grevgeny]

APPROVE — automated self-review of 093bbb6

No actionable findings. The deletion example and 204/404/409 behavior match the coordinated Core API revision; both source passes covered the full docs diff and connected API paths. Documentation whitespace checks and the examples build passed.

The Core change is being reviewed in ditioas/timestyr#5597. The new conflict behavior requires its deployment; cross-store cleanup failures can still require operational reconciliation.