#11995 fix(backend): remove dead Crisp unread endpoint

Closes #11429.

What changed and why

Remove the retired GET /v1/crisp/unread implementation while preserving the live screen-activity route on the same router. Remove its route-policy entry and record the intentional API removal in the app-client compatibility checker.

Product invariants affected

none

How it was verified

• PYTHONPATH=backend pytest -q backend/tests/unit/test_desktop_screen_crisp.py — 10 passed, including a route-level assertion that /v1/crisp/unread is now absent while the screen-activity suite remains green.
• make preflight — repository PR contracts passed.

Failure class (fixes)

Failure-Class: none