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_mqtt_live.py

@@ -9,7 +9,11 @@ from tests.live.conftest import assert_rfc5424
 @pytest.mark.live
 class TestMQTTLive:
     def test_connect_accepted(self, live_service):
-        port, drain = live_service("mqtt")
+        # The honeypot defaults to auth-required (post-2018 realistic
+        # broker posture). Opt into accept-all mode to exercise the
+        # happy-path CONNACK rc=0 code path. See decnet/templates/mqtt/
+        # server.py::MQTT_ACCEPT_ALL.
+        port, drain = live_service("mqtt", env={"MQTT_ACCEPT_ALL": "1"})
         connected = []
         client = mqtt.Client(client_id="test-scanner")
         client.on_connect = lambda c, u, f, rc: connected.append(rc)
@@ -48,7 +52,9 @@ class TestMQTTLive:
         )
 
     def test_subscribe_logged(self, live_service):
-        port, drain = live_service("mqtt")
+        # SUBSCRIBE is gated on successful auth — accept-all lets the test
+        # reach the subscribe path without planting credentials.
+        port, drain = live_service("mqtt", env={"MQTT_ACCEPT_ALL": "1"})
         subscribed = []
         client = mqtt.Client(client_id="sub-test")
         client.on_subscribe = lambda c, u, mid, qos: subscribed.append(mid)