fix: linearize alembic migrations

backend/tests/unit/test_alembic_migrations.py

@@ -0,0 +1,14 @@
+from pathlib import Path
+
+from alembic.config import Config
+from alembic.script import ScriptDirectory
+
+
+def test_alembic_has_single_head() -> None:
+    backend_dir = Path(__file__).resolve().parents[2]
+    config = Config(str(backend_dir / "alembic.ini"))
+    config.set_main_option("script_location", str(backend_dir / "alembic"))
+
+    script = ScriptDirectory.from_config(config)
+
+    assert script.get_heads() == ["0004"]