test(live): align mqtt/postgres/mysql live tests with honeypot + loop realities

Three unrelated test-correctness fixes exposed by running tests/live:

- test_mqtt_live: honeypot defaults to auth-required (post-2018
  realistic broker). Anonymous CONNECT is rejected with CONNACK rc=5,
  which the "accept" / "subscribe" tests misread as a failure. Pass
  MQTT_ACCEPT_ALL=1 via a new env= override on the live_service factory
  so only those two tests opt into accept-all.
- test_postgres_live::test_auth_hash_logged: connected with
  dbname='prod', which isn't in the honeypot's per-instance DB list, so
  Postgres (correctly) rejected at startup before asking for a
  password — blowing past the auth event the test asserts on. Target
  'postgres' (always in _BASE_DBS) to reach the auth stage.
- test_mysql_backend_live: the module-scoped mysql_test_db_url fixture
  is bound to the module loop, but function-scoped tests default to
  their own per-function loops. Any reuse of the asyncmy pool then
  tripped "Future attached to a different loop". Pin the whole module
  with pytest.mark.asyncio(loop_scope='module').

tests/live/test_mysql_backend_live.py

@@ -38,6 +38,13 @@ LIVE_URL = "mysql+asyncmy://root:root@127.0.0.1:3307/decnet"
 
 pytestmark = [
     pytest.mark.live,
+    # Pin every test in this module to the module-scoped event loop. The
+    # module-scoped ``mysql_test_db_url`` fixture (and transitively the
+    # asyncmy connection pool it seeds) is bound to that loop; running the
+    # tests on their own per-function loops trips pytest-asyncio's
+    # "Future attached to a different loop" guard the instant the repo
+    # reuses a pooled connection.
+    pytest.mark.asyncio(loop_scope="module"),
     pytest.mark.skipif(
         not (LIVE_URL and LIVE_URL.startswith("mysql")),
         reason="Set DECNET_DB_URL=mysql+aiomysql://... to run MySQL live tests",