feat: add default role management functionality

- Introduced a new default role service and repository for managing default roles assigned to new members.
- Added command to set the default role through the bot's configuration.
- Implemented event handling to assign the default role to new guild members upon joining.
- Updated main application to include the new default role service in the bot's services.

internal/commands/config/public/config.go

@@ -108,6 +108,19 @@ var Config = &discordgo.ApplicationCommand{
 				},
 			},
 		},
+		{
+			Type:        discordgo.ApplicationCommandOptionSubCommand,
+			Name:        "setdefaultrole",
+			Description: "Set the default role given to new members",
+			Options: []*discordgo.ApplicationCommandOption{
+				{
+					Type:        discordgo.ApplicationCommandOptionRole,
+					Name:        "role",
+					Description: "Role to assign on join",
+					Required:    true,
+				},
+			},
+		},
 	},
 }
 
@@ -153,6 +166,9 @@ func ConfigHandler(s *discordgo.Session, i *discordgo.InteractionCreate) {
 	case "setwelcomegif":
 		log.Printf("welcome: /config setwelcomegif invoked by %s in guild %s", i.Member.User.ID, i.GuildID)
 		handleSetWelcomeGIF(s, i)
+	case "setdefaultrole":
+		log.Printf("defaultrole: /config setdefaultrole invoked by %s in guild %s", i.Member.User.ID, i.GuildID)
+		handleSetDefaultRole(s, i)
 	default:
 		respondEphemeral(s, i, "Unknown subcommand.")
 	}
@@ -399,6 +415,51 @@ func handleSetWelcomeGIF(s *discordgo.Session, i *discordgo.InteractionCreate) {
 	respondEphemeral(s, i, "Welcome GIF updated.")
 }
 
+func handleSetDefaultRole(s *discordgo.Session, i *discordgo.InteractionCreate) {
+	if services.Global.DefaultRole == nil {
+		respondEphemeral(s, i, "Default role configuration is not available.")
+		return
+	}
+
+	guildID, err := strconv.ParseInt(i.GuildID, 10, 64)
+	if err != nil {
+		respondEphemeral(s, i, "Invalid guild ID.")
+		return
+	}
+
+	opt := i.ApplicationCommandData().Options[0]
+	var roleOpt *discordgo.ApplicationCommandInteractionDataOption
+	for _, o := range opt.Options {
+		if o.Name == "role" {
+			roleOpt = o
+			break
+		}
+	}
+	if roleOpt == nil {
+		respondEphemeral(s, i, "Missing role option.")
+		return
+	}
+
+	role := roleOpt.RoleValue(s, i.GuildID)
+	if role == nil {
+		respondEphemeral(s, i, "Invalid role.")
+		return
+	}
+
+	roleID, err := strconv.ParseInt(role.ID, 10, 64)
+	if err != nil {
+		respondEphemeral(s, i, "Invalid role ID.")
+		return
+	}
+
+	if err := services.Global.DefaultRole.SetRole(context.Background(), guildID, roleID); err != nil {
+		respondEphemeral(s, i, "Failed to set default role.")
+		return
+	}
+
+	respondEphemeral(s, i, "Default role set to "+role.Mention()+".")
+}
+
 func respondEphemeral(s *discordgo.Session, i *discordgo.InteractionCreate, msg string) {
 	_ = s.InteractionRespond(i.Interaction, &discordgo.InteractionResponse{
 		Type: discordgo.InteractionResponseChannelMessageWithSource,