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/audit/AuditFilters.tsx

@@ -5,21 +5,24 @@ import { useEffect, useState } from "react";
 import { FILTER_SELECT_CLASS } from "@/components/ui/Field";
 import { SearchInput } from "@/components/ui/SearchInput";
 
-const ACTIONS = [
-  "Neueinstellung",
-  "Wiedereinstellung",
-  "Rückkehr",
-  "Austritt",
-  "Versetzung",
-  "Ausschreibung",
-  "Interne Besetzung",
-  "Beförderung",
-  "Reorganisation",
-  "Reorganisation rückgängig",
-  "Karenz",
-  "Vertragsänderung",
-  "Stammdatenänderung",
-  "Gehaltsanpassung",
+// The value is the string stored in audit_log.action and must not change;
+// only what the user reads does — 'Karenz' is displayed as
+// "Langzeitabwesenheit" (see lib/absence.ts).
+const ACTIONS: { value: string; label: string }[] = [
+  { value: "Neueinstellung", label: "Neueinstellung" },
+  { value: "Wiedereinstellung", label: "Wiedereinstellung" },
+  { value: "Rückkehr", label: "Rückkehr aus Langzeitabwesenheit" },
+  { value: "Austritt", label: "Austritt" },
+  { value: "Versetzung", label: "Versetzung" },
+  { value: "Ausschreibung", label: "Ausschreibung" },
+  { value: "Interne Besetzung", label: "Interne Besetzung" },
+  { value: "Beförderung", label: "Beförderung" },
+  { value: "Reorganisation", label: "Reorganisation" },
+  { value: "Reorganisation rückgängig", label: "Reorganisation rückgängig" },
+  { value: "Karenz", label: "Langzeitabwesenheit" },
+  { value: "Vertragsänderung", label: "Vertragsänderung" },
+  { value: "Stammdatenänderung", label: "Stammdatenänderung" },
+  { value: "Gehaltsanpassung", label: "Gehaltsanpassung" },
 ];
 
 export function AuditFilters() {
@@ -60,8 +63,8 @@ export function AuditFilters() {
       >
         <option value="">Alle Aktionen</option>
         {ACTIONS.map((a) => (
-          <option key={a} value={a}>
-            {a}
+          <option key={a.value} value={a.value}>
+            {a.label}
           </option>
         ))}
       </select>