usernames: report reserved names as taken in the check paths too

account.checkUsername / channels.checkUsername / bots.checkUsername said a
reserved name was available and only updateUsername rejected it. Add the
blocklist check to peerUsernameAvailable (covers account + channel, both
backends) and to bots.Service.CheckUsername, so the client shows "username is
taken" immediately.

internal/store/memory/reserved_username_test.go

@@ -0,0 +1,37 @@
+package memory
+
+import (
+	"context"
+	"testing"
+
+	"telesrv/internal/domain"
+)
+
+func TestCheckUsernameReportsReservedAsTaken(t *testing.T) {
+	ctx := context.Background()
+	reserved := NewReservedUsernameStore()
+	if _, err := reserved.ReserveUsername(ctx, "support", "official", "ops"); err != nil {
+		t.Fatalf("seed reserve: %v", err)
+	}
+	registry := NewCollectibleUsernameStore().WithReservedUsernames(reserved)
+
+	users := NewUserStore()
+	users.AttachUsernameRegistry(registry)
+	u, _ := users.Create(ctx, domain.User{AccessHash: 1, Phone: "15550001000", FirstName: "A"})
+	if ok, err := users.CheckUsername(ctx, u.ID, "support"); err != nil || ok {
+		t.Fatalf("CheckUsername(reserved) = %v, %v; want false, nil", ok, err)
+	}
+	if ok, err := users.CheckUsername(ctx, u.ID, "freename"); err != nil || !ok {
+		t.Fatalf("CheckUsername(free) = %v, %v; want true, nil", ok, err)
+	}
+
+	channels := NewChannelStore()
+	channels.AttachUsernameRegistry(registry)
+	created, err := channels.CreateChannel(ctx, domain.CreateChannelRequest{CreatorUserID: u.ID, Title: "C", Megagroup: true, Date: 1})
+	if err != nil {
+		t.Fatalf("create channel: %v", err)
+	}
+	if ok, err := channels.CheckUsername(ctx, u.ID, created.Channel.ID, "support"); err != nil || ok {
+		t.Fatalf("channel CheckUsername(reserved) = %v, %v; want false, nil", ok, err)
+	}
+}