test: add provider postgres conformance

docs/development.md

@@ -37,6 +37,19 @@ The command runs:
 PYTHONPATH=src python3 -m unittest discover -s tests -p 'test_*.py'
 ```
 
+Live Postgres conformance tests are skipped by default. To run them against a
+dedicated disposable database, install `psycopg` or `psycopg2` in the active
+environment and set:
+
+```bash
+USER_ENGINE_POSTGRES_TEST_DSN='postgresql://...' \
+USER_ENGINE_POSTGRES_TEST_RESET=1 \
+make test
+```
+
+The reset flag is required because those tests delete rows from the
+bootstrap-owned `user_engine_*` tables.
+
 ## Implementation Rule
 
 Add new behavior in this order: