Put the deployment-only dependencies behind a server extra

A data-science environment installing fluksio waited for lxml, aiohttp and
the rest of a connector stack it has nothing to talk to. Outbound mail,
error reporting and the MQTT and InfluxDB clients moved to
`fluksio[server]`, which the image installs; each import is guarded and
names the extra. `tenacity` had no import site at all and is gone.

23 fewer packages and the compiled ones among them — a bare `pip install
fluksio` still serves, runs every python node, and registers the mqtt and
influxdb node types, which only need the library when one is actually
built. sentry-sdk arrives anyway underneath `fastapi[standard]`; what
changed there is that nothing of ours requires it.

The dev environment keeps every extra: the suite exercises the connectors
and strict mypy checks their call sites.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019Hra4ndWMCLU5F3KjUuVAc

backend/fluksio/flow/nodes/mqtt.py

@@ -20,6 +20,22 @@ if TYPE_CHECKING:
 logger = logging.getLogger(__name__)
 
 
+def _aiomqtt() -> Any:
+    """The client library, which is a `fluksio[server]` extra.
+
+    Imported per use rather than at module level, because the node type is
+    registered at boot and an installation that talks to no broker should not
+    have to carry the library to start.
+    """
+    try:
+        import aiomqtt
+    except ImportError:
+        raise RuntimeError(
+            "the mqtt node needs the server extra: pip install 'fluksio[server]'"
+        ) from None
+    return aiomqtt
+
+
 def topic_matches(filter_: str, topic: str) -> bool:
     """Does an MQTT topic filter cover this topic?
 
@@ -382,7 +398,7 @@ class MqttNode(Node):
         A dropped connection raises, and the supervisor decides when to
         reconnect — the same arrangement the subscriber uses.
         """
-        import aiomqtt
+        aiomqtt = _aiomqtt()
 
         queue = self._publish_queue
         if queue is None:
@@ -408,7 +424,7 @@ class MqttNode(Node):
 
     async def _publish_once(self, data: dict[str, Any]) -> None:
         """Connect, publish, disconnect — the unstarted node's path."""
-        import aiomqtt
+        aiomqtt = _aiomqtt()
 
         async with aiomqtt.Client(
             hostname=self.broker_host,
@@ -570,7 +586,7 @@ class MqttNode(Node):
         """
         import json
 
-        import aiomqtt
+        aiomqtt = _aiomqtt()
 
         if not (self._stop_event and self._stop_event.is_set()):
             try: