Say when a run finished and when a failure happened

`fluksio status` listed recent runs and recent failures with no time on
them, so a red line said nothing about whether it was from a minute ago or
last week. Both carry an age now, spelled the way a duration is, and the
runs listing gained one too. The fields were already on the wire.

backend/tests/test_cli.py

@@ -520,6 +520,24 @@ def test_two_files_of_one_name_are_refused(tmp_path) -> None:
         _import(*_module_of(second), second)
 
 
+def test_how_long_ago_reads_like_a_duration() -> None:
+    """A failure with no time on it says nothing about whether it is current."""
+    from datetime import UTC, datetime, timedelta
+
+    from fluksio.sdk.cli import _ago
+
+    def then(**delta):
+        return (datetime.now(UTC) - timedelta(**delta)).isoformat()
+
+    assert _ago(then(seconds=5)) == "5s ago"
+    assert _ago(then(minutes=3)) == "3min ago"
+    assert _ago(then(hours=2)) == "2h ago"
+    assert _ago(then(days=3)) == "3d ago"
+    # What the engine stores is UTC whether or not the spelling says so.
+    assert _ago(datetime.now(UTC).replace(tzinfo=None).isoformat()) == "0s ago"
+    assert _ago(None) == ""
+
+
 def test_a_serve_limit_is_refused_as_a_flag_not_as_a_traceback(capsys) -> None:
     """These are written into the environment before the settings are built."""
     import pytest