Multiple meeting lobbies and beta-gated features (3.5.2)

- /meeting add-lobby, remove-lobby, lobbies: up to 10 lobbies per server, capped in the database
- Deleted voice channels are dropped from the lobbies
- Beta features: registered only in the beta server and usable only with the Beta Tester role (BETA_GUILD_ID, BETA_ROLE_ID); fails closed
- /beta (bot owner only) shows how beta access is set up

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

main.go

@@ -6,6 +6,7 @@ import (
 	"os"
 	"os/signal"
 	"syscall"
+	"velox-bot/internal/beta"
 	"velox-bot/internal/bot"
 	"velox-bot/internal/commands"
 	cmdtickets "velox-bot/internal/commands/tickets"
@@ -45,6 +46,7 @@ func main() {
 		log.Fatalf("Error loading config: %v", err)
 		return
 	}
+	beta.Configure(config.BetaGuildID, config.BetaRoleID)
 
 	db, err := db.NewDB(config)
 	if err != nil {