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_postgres_live.py

@@ -60,13 +60,18 @@ class TestPostgresLive:
     def test_auth_hash_logged(self, live_service):
         port, drain = live_service("postgres")
         import psycopg2
+        # Real PG rejects before asking for a password when the requested
+        # db doesn't exist, and the honeypot faithfully mirrors that. So
+        # we must target an always-present database (``postgres`` is in
+        # _BASE_DBS) to get past startup and into the password-auth stage
+        # that this test is asserting on.
         try:
             psycopg2.connect(
                 host="127.0.0.1",
                 port=port,
                 user="root",
                 password="toor",
-                dbname="prod",
+                dbname="postgres",
                 connect_timeout=5,
             )
         except psycopg2.OperationalError: