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/EmployeeDetail.tsx

@@ -77,7 +77,7 @@ export function EmployeeDetail(props: EmployeeDetailProps) {
                 <h2 className="text-xl font-extrabold text-ink">
                   {fmtFullName(employee.first_name, employee.last_name, employee.title_prefix, employee.title_suffix)}
                 </h2>
-                <StatusChip status={employee.status} entryDate={employee.entry_date} />
+                <StatusChip status={employee.status} entryDate={employee.entry_date} absenceType={employee.absence_type} />
               </div>
               <p className="text-sm text-ink-body">{employee.job_title}</p>
               <p className="text-xs text-ink-muted">{breadcrumb}</p>
@@ -93,7 +93,11 @@ export function EmployeeDetail(props: EmployeeDetailProps) {
               <>
                 <ActionButton icon={ArrowRightLeft} label="Versetzen" onClick={() => setPanel("transfer")} />
                 <ActionButton icon={TrendingUp} label="Befördern" onClick={() => setPanel("promote")} />
-                <ActionButton icon={Clock} label={employee.status === "Karenz" ? "Karenz verwalten" : "Karenz"} onClick={() => setPanel("karenz")} />
+                <ActionButton
+                  icon={Clock}
+                  label={employee.status === "Karenz" ? "Abwesenheit verwalten" : "Langzeitabwesenheit"}
+                  onClick={() => setPanel("karenz")}
+                />
               </>
             )}
             {canEditData && <ActionButton icon={Pencil} label="Daten ändern" onClick={() => setPanel("daten")} />}