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.

tests/unit/absence.test.ts

@@ -0,0 +1,56 @@
+import { describe, expect, it } from "vitest";
+import { ABSENCE_TYPES, absenceLabel, isAbsenceType, statusLabel } from "@/lib/absence";
+
+// The stored employment status is still 'Karenz' — renaming the enum value
+// would mean rewriting every stored function that spells it. The rename
+// therefore lives entirely in this mapping, which makes it the one thing
+// standing between the database value and what an HR user reads.
+describe("statusLabel", () => {
+  it("renders the stored 'Karenz' value as Langzeitabwesenheit", () => {
+    expect(statusLabel("Karenz")).toBe("Langzeitabwesenheit");
+  });
+
+  it("leaves the other statuses alone", () => {
+    expect(statusLabel("Aktiv")).toBe("Aktiv");
+    expect(statusLabel("Geplant")).toBe("Geplant");
+    expect(statusLabel("Ausgetreten")).toBe("Ausgetreten");
+  });
+});
+
+describe("absenceLabel", () => {
+  it("shows the specific kind when one is recorded", () => {
+    expect(absenceLabel("Karenz", "Bildungskarenz")).toBe("Bildungskarenz");
+    expect(absenceLabel("Karenz", "Präsenzdienst")).toBe("Präsenzdienst");
+  });
+
+  it("falls back to the generic name for absences recorded before the field existed", () => {
+    expect(absenceLabel("Karenz", null)).toBe("Langzeitabwesenheit");
+    expect(absenceLabel("Karenz", "")).toBe("Langzeitabwesenheit");
+  });
+
+  it("ignores a type that is not on the list rather than displaying it", () => {
+    // The column has a check constraint, but the value arrives here as a
+    // plain string — echoing an unknown one would put unvalidated text in
+    // the status chip.
+    expect(absenceLabel("Karenz", "Ausgedachte Karenz")).toBe("Langzeitabwesenheit");
+  });
+
+  it("ignores the absence type entirely for any other status", () => {
+    expect(absenceLabel("Aktiv", "Sabbatical")).toBe("Aktiv");
+    expect(absenceLabel("Ausgetreten", "Papamonat")).toBe("Ausgetreten");
+  });
+});
+
+describe("ABSENCE_TYPES", () => {
+  it("holds the thirteen kinds, in the order they are offered", () => {
+    expect(ABSENCE_TYPES).toHaveLength(13);
+    expect(ABSENCE_TYPES[0]).toBe("Wochenhilfe (Mutterschutz)");
+    expect(ABSENCE_TYPES.at(-1)).toBe("Sabbatical");
+  });
+
+  it("guards every entry with isAbsenceType", () => {
+    for (const t of ABSENCE_TYPES) expect(isAbsenceType(t)).toBe(true);
+    expect(isAbsenceType("Karenz")).toBe(false);
+    expect(isAbsenceType(null)).toBe(false);
+  });
+});