[Story]: Offboarding checklist with AI aggregation (refines #119)

[adoLime]

User Story

As a Leaver, I want a personalized offboarding checklist that aggregates what I need to handle (owned runbooks, stale docs, open tickets), so that nothing critical gets lost.

Context & Motivation

• Type: Functional
• Parent epic: 10 EPIC - Quality Radar & Technical Debt Management #60 (10 EPIC – Quality Radar & Technical Debt Management / Knowledge Retention & Handover)
• Refines / supersedes: [Story]: Offboarding / leaver checklist #119 — this story implements [Story]: Offboarding / leaver checklist #119's full scope; [Story]: Offboarding / leaver checklist #119 stays open until this story is done
• Teams: team:backend, team:frontend, team:ai
• Dev guide: docs/handover-dev-guide-phase1-en.md § 4 (Story B) · Concept: docs/handover-feature-konzept.md § 7 + Anhang B (15-item core set with owner tags)
• Reusable: ArrivalStep/ArrivalStepState pattern, ArrivalStepsPage.tsx authoring scaffold

Acceptance Criteria

• DepartureStep/DepartureStepState (ArrivalStep pattern): stable key, company/project scope, settledBy, selfConfirmable, derivable steps
• Default set = trimmed core set, 15 items with owner tags (concept appendix B; handover-checklist.md remains the full reference)
• Authoring: PM/Admin edit, deactivate, add items (Admin system-wide incl. templates, PM per project)
• PM may deactivate company-wide steps project-locally (override, global set untouched)
• enabled instead of delete (process history kept); step visibility filtered server-side via audience
• Role add-ons as ProjectRole-bound step templates (Developer/Project Lead/Scrum Master)
• AI aggregation: "hands-on" items with artifact links (owned artifacts, stale docs, knowledge gaps, open tickets) — heuristic first, the LLM phrases it
• Each role checks off its own items (owner tag, enforced server-side via settledBy; doneBy logged); skip with reason → PM review; PM view: progress per leaver, blocked items

Sub-Tasks (by team)

Backend

• DepartureStep/DepartureStepState entities + repositories + Flyway migration
• Authoring CRUD + project-override endpoints (mirror Arrival authoring)
• settledBy-enforced confirm endpoints (leaver/PM/receiver) + skip-with-reason
• PM progress metrics per leaver
• Tests (MockK/AssertJ) incl. RBAC, override isolation, audience filtering

Frontend

• Leaver checklist UI (phase tabs + progress, reuse features/onboarding/ components)
• Authoring UI (reuse ArrivalStepsPage scaffold: scope tabs, override toggle)
• PM progress view

AI

• POST /handover/checklist/prioritize (heuristic first, LLM phrasing) + ScriptedLLMClient tests

Dependencies

A → B → C → D (B/C can overlap after A, D needs C). Builds on the UserLeavingEvent listener groundwork from Story A.