Beta servers: testers can use beta features in servers whose owner opted in

- beta.Configure takes the opted-in servers; testers are looked up in the beta server
- beta commands registered per opted-in server and removed when it opts out
- beta.ServerEnabled for behaviour that no person triggers
- docs updated

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

internal/db/repos/settingsrepo/meeting_integration_test.go

@@ -180,3 +180,38 @@ func TestGetBetaSettings(t *testing.T) {
 		t.Fatalf("role = %q, want %q", role, "42")
 	}
 }
+
+func TestListBetaGuilds(t *testing.T) {
+	r, _ := testRepo(t)
+	db := r.db
+	ctx := context.Background()
+	if _, err := db.ExecContext(ctx, `CREATE TABLE IF NOT EXISTS beta_guilds (
+		guild_id BIGINT PRIMARY KEY, enabled_by BIGINT NOT NULL, enabled_at TIMESTAMPTZ NOT NULL DEFAULT NOW())`); err != nil {
+		t.Fatal(err)
+	}
+	const g = 8800000000000004242
+	_, _ = db.ExecContext(ctx, `DELETE FROM beta_guilds WHERE guild_id = $1`, g)
+	t.Cleanup(func() { _, _ = db.ExecContext(ctx, `DELETE FROM beta_guilds WHERE guild_id = $1`, g) })
+
+	has := func() bool {
+		ids, err := r.ListBetaGuilds(ctx)
+		if err != nil {
+			t.Fatal(err)
+		}
+		for _, id := range ids {
+			if id == "8800000000000004242" {
+				return true
+			}
+		}
+		return false
+	}
+	if has() {
+		t.Fatal("not listed before opting in")
+	}
+	if _, err := db.ExecContext(ctx, `INSERT INTO beta_guilds (guild_id, enabled_by) VALUES ($1, 1)`, g); err != nil {
+		t.Fatal(err)
+	}
+	if !has() {
+		t.Fatal("listed after opting in")
+	}
+}