feat(plan): assertNotPastDate guard for write paths

src/__tests__/plan.test.ts

@@ -4,6 +4,7 @@ import {
   resolveCell,
   resolveGrid,
   listPlanUsers,
+  assertNotPastDate,
 } from "../services/plan.service";
 
 // ---------------------------------------------------------------------------
@@ -187,3 +188,28 @@ describe("plan.service.listPlanUsers", () => {
     expect(found?.has_attendance_record).toBe(true);
   });
 });
+
+// ---------------------------------------------------------------------------
+// assertNotPastDate
+// ---------------------------------------------------------------------------
+
+describe("plan.service.assertNotPastDate", () => {
+  it("returns error for a past date", () => {
+    const result = assertNotPastDate("2000-01-01", false);
+    expect(result).toEqual({
+      error:
+        "Nelze upravovat plán pro datum v minulosti. Pro nouzovou opravu použijte ?force=1.",
+      status: 403,
+    });
+  });
+
+  it("returns null for a future date", () => {
+    const result = assertNotPastDate("2099-01-01", false);
+    expect(result).toBeNull();
+  });
+
+  it("returns null for a past date with force=true", () => {
+    const result = assertNotPastDate("2000-01-01", true);
+    expect(result).toBeNull();
+  });
+});