Beta: clean up servers that opted out while the bot was offline

The bot remembers where it registered beta commands (beta_registered) and
removes them from servers that are no longer in the beta at the next start.
Failed cleanups are retried; a server the bot has left is forgotten.

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>

internal/db/repos/settingsrepo/meeting_integration_test.go

@@ -215,3 +215,44 @@ func TestListBetaGuilds(t *testing.T) {
 		t.Fatal("listed after opting in")
 	}
 }
+
+func TestBetaRegistered(t *testing.T) {
+	r, _ := testRepo(t)
+	ctx := context.Background()
+	if _, err := r.db.ExecContext(ctx, `CREATE TABLE IF NOT EXISTS beta_registered (guild_id BIGINT PRIMARY KEY)`); err != nil {
+		t.Fatal(err)
+	}
+	const g = "8800000000000005151"
+	_ = r.SetBetaRegistered(ctx, g, false)
+	t.Cleanup(func() { _ = r.SetBetaRegistered(ctx, g, false) })
+
+	has := func() bool {
+		ids, err := r.ListBetaRegistered(ctx)
+		if err != nil {
+			t.Fatal(err)
+		}
+		for _, id := range ids {
+			if id == g {
+				return true
+			}
+		}
+		return false
+	}
+	if has() {
+		t.Fatal("not remembered at first")
+	}
+	for i := 0; i < 2; i++ { // twice: harmless
+		if err := r.SetBetaRegistered(ctx, g, true); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if !has() {
+		t.Fatal("remembered after registering")
+	}
+	if err := r.SetBetaRegistered(ctx, g, false); err != nil || has() {
+		t.Fatalf("forgotten after removing: %v", err)
+	}
+	if err := r.SetBetaRegistered(ctx, "not-a-number", true); err == nil {
+		t.Error("a bad ID is refused")
+	}
+}