Rename Karenz to Langzeitabwesenheit and record its type

Karenz was doing duty as the name for every kind of extended absence, but
the cases behave differently in payroll and reporting — Wochenhilfe, a
Präsenzdienst, a long sick leave and a sabbatical are not the same thing.
The concept is now called Langzeitabwesenheit and carries which kind it is.

- employees.absence_type, constrained to the thirteen kinds. start_karenz
  stores it on both paths (written straight away, or parked in the
  pending_org_changes payload when the absence starts later);
  record_karenz_return and the karenz_return branch of
  apply_due_pending_changes clear it, so a returned employee does not keep
  looking like they are still away. It also reaches employee_history, the
  audit log and the employee export.
- The status enum value stays 'Karenz'. Postgres can rename an enum value in
  place, but every stored function body that spells it would then reference
  a value that no longer exists — a dozen functions across fifteen
  migrations, rewritten for a label. The mapping lives in lib/absence.ts
  instead, which is the single place the UI reads the display name from.
- Where a kind is recorded the chip shows it — "Bildungskarenz" says more
  than "Langzeitabwesenheit". Absences predating the field have none and
  fall back to the generic name rather than to a guess, and a value outside
  the list is dropped rather than echoed into the UI.
- The export prints the display name, not the raw enum: a payroll hand-off
  reading "Karenz" for what the app calls Langzeitabwesenheit only causes
  questions. Audit filter options keep their stored values and change only
  their labels.
- The seed spreads the twelve absences across the kinds; all of them being
  Karenz would leave any breakdown by kind invisible.

components/employees/EmployeeFilters.tsx

@@ -10,13 +10,14 @@ type EmployeeFiltersProps = {
   locations: { id: string; name: string }[];
 };
 
-// Aktiv and Karenz are separate statuses — somebody on Karenz is employed
-// but not active. The combined entry is offered explicitly, and named after
-// the two statuses it selects rather than calling the pair "aktiv".
+// Aktiv and Langzeitabwesenheit are separate statuses — somebody on a
+// long-term absence is employed but not active. The combined entry is
+// offered explicitly, named after the two it selects rather than calling
+// the pair "aktiv". The stored value is still 'Karenz' (see lib/absence.ts).
 const STATUS_OPTIONS = [
   { value: "Aktiv", label: "Aktiv" },
-  { value: "Karenz", label: "Karenz" },
-  { value: "Aktiv,Karenz", label: "Aktiv + Karenz (beschäftigt)" },
+  { value: "Karenz", label: "Langzeitabwesenheit" },
+  { value: "Aktiv,Karenz", label: "Aktiv + Langzeitabwesenheit (beschäftigt)" },
   { value: "Geplant", label: "Geplant" },
   { value: "Ausgetreten", label: "Ausgetreten" },
 ] as const;