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.

lib/reports.ts

@@ -329,14 +329,15 @@ export const EVENT_TYPE_LABELS: Record<HistoryEventType, string> = {
   Eintritt: "Eintritt",
   Beförderung: "Beförderung",
   Versetzung: "Versetzung",
-  Karenz: "Karenz",
+  // Stored value stays 'Karenz'; the label follows the renamed concept.
+  Karenz: "Langzeitabwesenheit",
   Vertragsänderung: "Vertragsänderung",
   Stammdatenänderung: "Stammdatenänderung",
   Austritt: "Austritt",
   Wiedereintritt: "Wiedereintritt",
   Reorganisation: "Reorganisation",
   Gehaltsanpassung: "Gehaltsanpassung",
-  Rückkehr: "Rückkehr (Karenz)",
+  Rückkehr: "Rückkehr aus Langzeitabwesenheit",
 };
 
 export type ReportEvent = {