fix(workbench): configure psycopg snapshot transaction

Open the psycopg transaction without unsupported SQLAlchemy-style options, then set the PostgreSQL isolation and read-only characteristics before the first query. Cover the exact snapshot endpoint path with a regression test.

tests/unit/test_workbench_events.py

@@ -1,4 +1,5 @@
 from contextlib import asynccontextmanager
+from datetime import datetime, timezone
 from types import SimpleNamespace
 from uuid import uuid4
 
@@ -43,6 +44,77 @@ class FakeMetadataRepository:
         return self.routings
 
 
+class FakeSnapshotConnection:
+    def __init__(self):
+        self.statements = []
+
+    @asynccontextmanager
+    async def transaction(self):
+        yield
+
+    async def execute(self, statement):
+        self.statements.append(statement)
+
+
+class FakeEventRepository:
+    def __init__(self, _conn, _project_id):
+        pass
+
+    async def current_cursor(self):
+        return 12
+
+
+class FakeOperationalTaskService:
+    async def list(self, **_kwargs):
+        return [], None
+
+    async def list_notifications(self, **_kwargs):
+        return [], None
+
+
+@pytest.mark.anyio
+async def test_sync_snapshot_starts_psycopg_read_only_repeatable_read_transaction(monkeypatch):
+    project_id = uuid4()
+    conn = FakeSnapshotConnection()
+    now = datetime.now(timezone.utc)
+
+    async def fake_runtime(**_kwargs):
+        return {
+            "availability": "not_configured",
+            "source": "none",
+            "scenario_availability": "not_configured",
+            "artifact_availability": "not_configured",
+            "schedule_availability": "not_configured",
+            "work_order_availability": "not_configured",
+            "alert_availability": "not_configured",
+            "generated_at": now,
+            "conditions": [],
+            "alerts": [],
+            "scenarios": [],
+            "latest_analysis": None,
+            "work_orders": [],
+        }
+
+    monkeypatch.setattr(endpoint, "WorkbenchEventRepository", FakeEventRepository)
+    monkeypatch.setattr(endpoint, "workbench_runtime", fake_runtime)
+    monkeypatch.setattr(endpoint, "_service", lambda _ctx, _conn: FakeOperationalTaskService())
+
+    snapshot = await endpoint.workbench_sync_snapshot(
+        ctx=ProjectContext(
+            project_id=project_id,
+            project_code="project-a",
+            user_id=uuid4(),
+            project_role="member",
+        ),
+        conn=conn,
+    )
+
+    assert snapshot.event_cursor == 12
+    assert conn.statements == [
+        "SET TRANSACTION ISOLATION LEVEL REPEATABLE READ READ ONLY"
+    ]
+
+
 @pytest.mark.anyio
 async def test_expired_event_cursor_requires_snapshot_reset(monkeypatch):
     project_id = uuid4()