Feat: add add_tribe in service

backend/service/services.py

@@ -0,0 +1,16 @@
+from backend.model.tribe import Tribe
+from backend.repository.abstract_repository import AbstractRepository
+from backend.repository.tribe_sqlite_repository import TribeRepositoryError
+
+
+class TribeExists(Exception):
+    pass
+
+
+def add_tribe(tribe: Tribe, tribe_repo: AbstractRepository, conn):
+    try:
+        tribe_repo.add(tribe)
+    except TribeRepositoryError:
+        raise TribeExists(f"The tribe {tribe.name} already exists")
+    conn.commit()
+    return tribe