fix: handle concurrent table creation race in SQLite (#151)

tests/dashboard/test_calm.py

@@ -235,3 +235,22 @@ def test_reorder_promote_later_to_next(client: TestClient, db_session: Session):
     assert db_session.query(Task).filter(Task.id == task_now.id).first().state == TaskState.NOW
     assert db_session.query(Task).filter(Task.id == task_later1.id).first().state == TaskState.NEXT
     assert db_session.query(Task).filter(Task.id == task_later2.id).first().state == TaskState.LATER
+
+
+def test_create_tables_idempotent_under_concurrency():
+    """Calling create_tables() when tables already exist must not crash.
+
+    This covers the race where multiple pytest-xdist workers (or app
+    processes) import the calm routes module simultaneously and each
+    calls create_tables() against the same SQLite file.
+    """
+    from unittest.mock import patch
+
+    from sqlalchemy.exc import OperationalError
+
+    from dashboard.models.database import create_tables
+
+    fake_error = OperationalError("CREATE TABLE", {}, Exception("table tasks already exists"))
+    with patch("dashboard.models.database.Base.metadata.create_all", side_effect=fake_error):
+        # Must not raise — the OperationalError is caught and logged
+        create_tables()