fix: improve Change Cases workflow UX

[DeepZone]

Motivation

• Improve the Change Cases user experience and lifecycle gaps introduced after v0.7.0-beta by replacing browser prompts and adding meaningful UI actions.
• Provide safe deletion semantics so operators/admins can remove cases while preserving checks/reports and keeping a clear audit trail.
• Enable running checks from a selected case (no hardcoded AS3320) and surface report details for reviewers and viewers.

Description

• Added backend DELETE /api/change-cases/{id} which is allowed for operator/admin, detaches checks by setting checks.change_case_id = NULL, preserves reports/checks, deletes the case, emits change_case_deleted audit events, and returns {"ok": true, "detached_checks": n}.
• Added backend tests covering admin/operator deletion, viewer denial, detachment behavior, report-preservation, and audit event emission.
• Extended frontend API with deleteChangeCase(id), runPrefixCheck(...), and runPreflightCheck(...), and made checkPrefix/checkPreflight accept an optional change_case_id for backward compatibility.
• Reworked Change Cases UI to use an in-app create form (title/description), detail edit mode, status-badge and workflow actions, ASN/Prefix/Preflight check forms that send change_case_id, a reports table with action buttons, and a role-gated delete button with confirmation and clear feedback.
• Added a v0.7.0-beta hotfix: Change Cases UX polish entry to RELEASE_NOTES.md without bumping to v0.7.1-beta.

Testing

• Ran backend test suite with cd backend && pytest -q and all tests passed (94 passed).
• Built the frontend with cd frontend && npm run build and the production build completed successfully.
• New backend tests for deletion/detachment and audit were executed as part of the test run and passed.

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Codex Task