fix: select existing tenants by name in the operator portal

Assistant: codex
Assistant-Model: gpt-6-astra
Assistant-Session: 01a07ff8-19d0-7820-b4d0-1353833cb7fc

src/user_engine/adapters/postgres.py

@@ -157,6 +157,18 @@ class PostgresUserEngineStore:
             self._query_records("memberships", tenant=tenant),
         )
 
+    def membership_tenants(self) -> tuple[str, ...]:
+        with self._cursor() as cursor:
+            cursor.execute(
+                "SELECT DISTINCT tenant FROM user_engine_records "
+                "WHERE record_type = %s AND tenant IS NOT NULL ORDER BY tenant",
+                ("memberships",),
+            )
+            return tuple(
+                str(row["tenant"] if isinstance(row, Mapping) else row[0])
+                for row in cursor.fetchall()
+            )
+
     def save_application(self, application: Application) -> None:
         self._upsert_record(application)