Give the Influx and MQTT nodes their two missing knobs

The Influx client was built with no timeout, so every query and write fell
through to influxdb-client's own 10 s default — invisible to a flow and
unchangeable. The param is in seconds like its peers; the client counts in
milliseconds, so the call sites convert.

The publisher backlog was a module constant, read once at import. It is the
depth at which the oldest payload is dropped and the node goes degraded, and
a node that bursts wants more than one that trickles, so it moves to Params
and is read where the queue is built.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K1moruzue2kTJd3uVisgNk

backend/tests/flow/test_senders.py

@@ -119,3 +119,35 @@ def test_the_configured_timeout_reaches_the_broker_client(monkeypatch):
     asyncio.run(node._publish_once({"setpoint": 21.0}))
 
     assert seen["timeout"] == 2.5
+
+
+def test_the_configured_backlog_reaches_the_publish_queue(monkeypatch):
+    """The depth is read when the queue is built, so it has to be per node."""
+    import aiomqtt
+
+    class FakeClient:
+        def __init__(self, **kwargs):
+            pass
+
+        async def __aenter__(self):
+            return self
+
+        async def __aexit__(self, *_):
+            return False
+
+    monkeypatch.setattr(aiomqtt, "Client", FakeClient)
+
+    node = MqttNode(
+        requires=[MessageSpec(name="setpoint", port="setpoint", dtype=DType.FLOAT)],
+        params={"topic": {"setpoint": "heating/setpoint"}, "publish_queue_size": 8},
+    )
+    node.assign_flow("heating", "out")
+
+    async def scenario() -> int:
+        await node.start_publisher()
+        assert node._publish_queue is not None
+        size = node._publish_queue.maxsize
+        await node.stop_publisher()
+        return size
+
+    assert asyncio.run(scenario()) == 8