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/db/repos/defaultrolerepo/repo.go

@@ -0,0 +1,47 @@
+package defaultrolerepo
+
+import (
+	"context"
+	"database/sql"
+)
+
+type Repo struct {
+	db *sql.DB
+}
+
+func NewRepo(db *sql.DB) *Repo {
+	return &Repo{db: db}
+}
+
+// GetRole returns the configured default role ID for a guild, or 0 if none.
+func (r *Repo) GetRole(ctx context.Context, guildID int64) (int64, error) {
+	const q = `
+		SELECT role_id
+		FROM defaultrole
+		WHERE guild_id = $1
+	`
+	var roleID sql.NullInt64
+	if err := r.db.QueryRowContext(ctx, q, guildID).Scan(&roleID); err != nil {
+		if err == sql.ErrNoRows {
+			return 0, nil
+		}
+		return 0, err
+	}
+	if !roleID.Valid {
+		return 0, nil
+	}
+	return roleID.Int64, nil
+}
+
+// SetRole upserts the default role for a guild.
+func (r *Repo) SetRole(ctx context.Context, guildID, roleID int64) error {
+	const q = `
+		INSERT INTO defaultrole (guild_id, role_id)
+		VALUES ($1, $2)
+		ON CONFLICT (guild_id)
+		DO UPDATE SET role_id = EXCLUDED.role_id
+	`
+	_, err := r.db.ExecContext(ctx, q, guildID, roleID)
+	return err
+}
+