fix(download): make "check now" actually check now, and say what it did

The button ran a normal reconcile pass, which honours each request's
retry backoff — so a request searched an hour ago was not due, nothing
was searched, and the button looked broken. The backoff is a promise to
the providers, not to the user: a person pressing "check now" *is* the
schedule, so a user-initiated pass ignores it and the loop still does
not.

"Nothing happened" also needed a reason. Summary now carries how many
requests are still being looked for and whether any download client is
enabled at all, which is the one cause of silence the user can fix —
and the requests tab says so above the list rather than leaving an
inert list to be interpreted.

The rest is the retry schedule finally being admitted to: rows show
when the next check falls due, "Looking for" explains that a request
sitting there is waiting rather than failing, and the page header says
how often the list is worked.

backend/download/reconcile_test.go

@@ -495,3 +495,101 @@ func waitFor(t *testing.T, cond func() bool, msg string) {
 
 	t.Fatal(msg)
 }
+
+// "Check now" is the user overriding the retry schedule, so it must
+// search a request whose backoff has not elapsed.  The scheduled pass
+// must not: the backoff exists to keep a fruitless search off the
+// providers, and a loop that ignored it would hammer them.
+func TestRunNowIgnoresBackoffAndRunOnceDoesNot(t *testing.T) {
+	t.Parallel()
+
+	f := newReconcileFixture(t)
+	ctx := context.Background()
+
+	provider := NewFakeProvider(1, "weak", Caps{CanSearch: true, CanTransport: true})
+	provider.Candidates = []Candidate{candidateFor(
+		"weak-1", []string{"Something Else Entirely"}, ".mp3", 3_000_000,
+	)}
+
+	f.manager.installProvider(Config{ID: 1, Priority: 50}, provider)
+
+	id, err := f.store.AddRequest(ctx, Request{
+		MBID:      "rg-1",
+		Entity:    EntityReleaseGroup,
+		LibraryID: 1,
+		Artist:    "Radiohead",
+		Title:     "OK Computer",
+	})
+	if err != nil {
+		t.Fatalf("AddRequest: %v", err)
+	}
+
+	f.catalog.tracklists["rg-1"] = fourTrackDownload().Expected
+
+	// First pass: attempted, found nothing, backoff armed.
+	if _, err := f.reconciler.RunOnce(ctx); err != nil {
+		t.Fatalf("RunOnce: %v", err)
+	}
+
+	scheduled, err := f.reconciler.RunOnce(ctx)
+	if err != nil {
+		t.Fatalf("RunOnce (second): %v", err)
+	}
+
+	if scheduled.Attempted != 0 {
+		t.Errorf("scheduled pass attempted %d, want 0 while backed off",
+			scheduled.Attempted)
+	}
+
+	forced, err := f.reconciler.RunNow(ctx)
+	if err != nil {
+		t.Fatalf("RunNow: %v", err)
+	}
+
+	if forced.Attempted != 1 {
+		t.Errorf("forced pass attempted %d, want 1", forced.Attempted)
+	}
+
+	if forced.Waiting != 1 {
+		t.Errorf("summary reported %d waiting, want 1 so the UI can say "+
+			"what was searched", forced.Waiting)
+	}
+
+	req, err := f.store.GetRequest(ctx, id)
+	if err != nil {
+		t.Fatalf("GetRequest: %v", err)
+	}
+
+	if req.Attempts != 2 {
+		t.Errorf("attempts = %d, want 2 after a forced re-check", req.Attempts)
+	}
+}
+
+// A pass with no providers says so, because "nothing happened" with no
+// reason is the one outcome the user cannot act on.
+func TestSummaryReportsNoProviders(t *testing.T) {
+	t.Parallel()
+
+	f := newReconcileFixture(t)
+	ctx := context.Background()
+
+	if _, err := f.store.AddRequest(ctx, Request{
+		MBID:      "rg-1",
+		Entity:    EntityReleaseGroup,
+		LibraryID: 1,
+		Title:     "OK Computer",
+	}); err != nil {
+		t.Fatalf("AddRequest: %v", err)
+	}
+
+	f.catalog.tracklists["rg-1"] = fourTrackDownload().Expected
+
+	summary, err := f.reconciler.RunNow(ctx)
+	if err != nil {
+		t.Fatalf("RunNow: %v", err)
+	}
+
+	if !summary.NoProviders {
+		t.Error("summary did not report that no download client is enabled")
+	}
+}