Fix: Validate source_date in copy availability endpoint

Added validation to ensure source_date is within the allowed range
(tomorrow to +30 days) for consistency with target_dates validation.

backend/tests/test_availability.py

@@ -524,3 +524,17 @@ class TestCopyAvailability:
         assert response.status_code == 400
         assert "30" in response.json()["detail"]
 
+    @pytest.mark.asyncio
+    async def test_copy_validates_source_date(self, client_factory, admin_user):
+        """Cannot copy from a past source date."""
+        async with client_factory.create(cookies=admin_user["cookies"]) as client:
+            response = await client.post(
+                "/api/admin/availability/copy",
+                json={
+                    "source_date": str(date.today() - timedelta(days=1)),  # Yesterday
+                    "target_dates": [str(in_days(1))],
+                },
+            )
+        
+        assert response.status_code == 400
+        assert "past" in response.json()["detail"].lower()