Roll reporting up past an absent manager, and say so on both sides

While somebody is on a long-term absence their reports report to the next
management level, and it keeps rolling up until it reaches somebody present.
Derived at read time in lib/acting-manager.ts rather than written to
employees.manager_id: the absent person stays formally in charge, so the
stand-in has to be visible as a stand-in rather than quietly replacing them.
Both ids therefore travel to the UI, and both sides carry a badge — the
absent person ("Abwesend · Vertretung: X") and anyone now reporting
elsewhere ("Vertretung für Y").

Three cases the walk has to survive, all covered by tests:
- Several absent levels in a row — it keeps climbing, and still names the
  *recorded* manager as the one being covered for, not the level skipped.
- Everyone above absent — it stops and keeps the recorded manager. Re-rooting
  a team to the top of the chart would distort more than showing an absent
  manager whose absence is labelled anyway.
- A manager_id cycle, which nothing in the schema forbids.

An absent lead needs no separate deputy field: their stand-in is simply
their own acting manager, the same one their reports moved to.

tests/unit/orgchart-data.test.ts

@@ -24,6 +24,7 @@ function emp(id: string, overrides: Partial<EmployeeInput> = {}): EmployeeInput
     exit_date: null,
     karenz_start_date: null,
     karenz_return_date: null,
+    absence_type: null,
     ...overrides,
   };
 }