Drive the archive reopen debounce test from a clock (closes #190)

The archive writer now reads the time its reopen debounce is
measured on from a clock field, time.Now outside tests. The
debounce test moves that clock instead of sleeping past a real
2 second window, so a slow host between the two quick writes can
no longer turn a correct result red.

Model: opus-5-5

internal/delivery/target_database_test.go

@@ -184,13 +184,20 @@ func TestArchiveWriter_RecreatesAfterRemoval(
 func TestArchiveWriter_ReopenDebounce(t *testing.T) {
 	t.Parallel()
 
-	// A generous debounce keeps the two rapid writes inside
-	// the window even on a heavily loaded test machine.
+	const debounce = 2 * time.Second
+
 	path := filepath.Join(t.TempDir(), "archive-wh.db")
 	w := delivery.NewExportArchiveWriter(
-		path, archiveTestLogger(), 2*time.Second,
+		path, archiveTestLogger(), debounce,
 	)
 
+	// The writer reads the time from this clock, which only the
+	// test moves, so how long the host takes between writes
+	// cannot change the result.
+	now := time.Now()
+
+	w.SetNow(func() time.Time { return now })
+
 	require.NoError(t, w.Write(
 		delivery.ExportArchivedEvent{EventID: "a"}, 0,
 	))
@@ -202,7 +209,7 @@ func TestArchiveWriter_ReopenDebounce(t *testing.T) {
 	// initial open — no extra close/reopen.
 	assert.Equal(t, 1, w.Reopens())
 
-	time.Sleep(2100 * time.Millisecond)
+	now = now.Add(debounce)
 
 	require.NoError(t, w.Write(
 		delivery.ExportArchivedEvent{EventID: "c"}, 0,