feat: add ServiceError and detached-safe session factory

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_db.py

@@ -123,3 +123,16 @@ def test_pending_migrations_applied_in_order_once(tmp_path, monkeypatch):
     assert _user_version(engine) == 3
     migrate(engine)  # re-run is a no-op
     assert applied == [2, 3]
+
+
+def test_rows_stay_readable_after_session_closes(tmp_path):
+    # Services use per-operation sessions; rows they return must remain
+    # readable after the originating session closes (expire_on_commit=False).
+    settings = _tmp_settings(tmp_path)
+    ensure_db(settings)
+    factory = get_session_factory(get_engine(settings))
+    with factory() as session:
+        p = Project(name="x", path="/x")
+        session.add(p)
+        session.commit()
+    assert p.name == "x"  # would raise DetachedInstanceError without the flag