U1: Bump _SCHEMA_VERSION to 3, add 5 department tables (departments,
user_departments, department_skill_bindings, department_kb_bindings,
department_quotas) + 5 ORM models + helpers.
U2: DepartmentService (12 async methods: CRUD + bind/unbind skill/KB +
count_users). Mount admin_router in app.py. 36 unit + 28 integration tests.
U4: DepartmentContext FastAPI dependency (per-route, admin bypasses
filtering). filter_skills_by_department / filter_kb_sources_by_department
helpers. Applied to GET /skills and GET /kb-management/* routes.
15 integration tests for department isolation.
Also includes brainstorm + plan docs. 108 new tests, all pass.
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