test(downloads): guard the service fixture on something the fake sets

`newServiceFixture` stops auto-pick from starting a grab, because none
of its tests is about the download and a detached `go m.grab(...)`
racing `t.TempDir()`'s cleanup is how they fail. It did that with
`MaxSizeMB: 1` -- and the size gates read `Candidate.TotalSize`, which
real providers fill and the fake leaves at zero. Zero is under every
ceiling, so the guard never fired and the race it was written to
prevent kept happening, roughly one run in fifteen:

    TempDir RemoveAll cleanup: unlinkat ... : directory not empty

The guard is a format the fake never produces. Thirty consecutive
whole-package runs, none.

`TestManualDownloadSatisfiesRequestOnSuccess` was relying on the guard
being broken -- it is the one test here that wants the download -- so
it now clears the preferences itself rather than depending on a bug.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MeQt5hgXg5YGoNZQ9ozG7L

backend/download/service_test.go

@@ -36,13 +36,24 @@ func newServiceFixture(t *testing.T) serviceFixture {
 	// assertion read it; the second is that same goroutine still writing
 	// into `t.TempDir()` after the test returned. One cause, two shapes.
 	//
-	// Putting the candidate outside the auto-pick size window stops the
+	// Putting the candidate outside the auto-pick guardrails stops the
 	// grab from ever starting, which is better than waiting for it: there
 	// is no goroutine to be slow, so the tests state what they mean
 	// ("the request exists, in this state") without a timing assumption
 	// underneath. A test that does want the download has `managerFixture`
 	// and sets its own preferences.
-	mf.manager.SetPreferences(AutoDownloadPrefs{MaxSizeMB: 1})
+	//
+	// The guard is a *format* the fake never produces, and it used to be
+	// `MaxSizeMB: 1`, which never fired: the size gates read
+	// `Candidate.TotalSize`, which real providers fill and the fake
+	// leaves at zero, and zero is under every ceiling. So the grab went
+	// ahead anyway and the second failure shape above — the TempDir
+	// cleanup race — kept happening, reproducibly, roughly one run in
+	// fifteen. A guard has to be keyed on something the fixture
+	// actually sets.
+	mf.manager.SetPreferences(AutoDownloadPrefs{
+		AllowedFormats: []Format{FormatWMA},
+	})
 
 	return serviceFixture{managerFixture: mf, svc: svc}
 }
@@ -182,6 +193,11 @@ func TestManualDownloadSatisfiesRequestOnSuccess(t *testing.T) {
 	f := newServiceFixture(t)
 	ctx := context.Background()
 
+	// This is the one test here that is *about* the download, so it
+	// undoes the fixture's guard rather than relying on it — which is
+	// what it was doing implicitly while the guard did not work.
+	f.manager.SetPreferences(AutoDownloadPrefs{})
+
 	provider := fakeWithAlbum(1, "source", ".flac")
 	f.manager.installProvider(Config{ID: 1, Priority: 50}, provider)