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.

app/(app)/page.tsx

@@ -33,7 +33,7 @@ const DOT_STYLES: Record<string, string> = {
   Gehaltsanpassung: "bg-warning-text",
 };
 
-const KIND_LABEL = { hire: "Eintritt", exit: "Austritt", return: "Karenz-Rückkehr" } as const;
+const KIND_LABEL = { hire: "Eintritt", exit: "Austritt", return: "Rückkehr aus Abwesenheit" } as const;
 
 export default async function DashboardPage() {
   const supabase = await createClient();
@@ -213,7 +213,7 @@ export default async function DashboardPage() {
       tone: "danger",
       href: `/reports?mode=events&eventType=Austritt&from=${yearStart}&to=${yearEnd}`,
     },
-    { label: "In Karenz", value: karenzCount, tone: "warning", href: "/employees?status=Karenz" },
+    { label: "Langzeitabwesend", value: karenzCount, tone: "warning", href: "/employees?status=Karenz" },
     { label: "Offene Positionen", value: openPositionsRes.count ?? 0, tone: "brand", href: "/positions" },
   ];