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/services/defaultrole/service.go

@@ -0,0 +1,24 @@
+package defaultrole
+
+import (
+	"context"
+
+	"velox-bot/internal/db/repos/defaultrolerepo"
+)
+
+type Service struct {
+	repo *defaultrolerepo.Repo
+}
+
+func New(repo *defaultrolerepo.Repo) *Service {
+	return &Service{repo: repo}
+}
+
+func (s *Service) GetRole(ctx context.Context, guildID int64) (int64, error) {
+	return s.repo.GetRole(ctx, guildID)
+}
+
+func (s *Service) SetRole(ctx context.Context, guildID, roleID int64) error {
+	return s.repo.SetRole(ctx, guildID, roleID)
+}
+