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/influx.py

@@ -14,6 +14,22 @@ from fluksio.flow.nodes.base import Node, NodeResult
 logger = logging.getLogger(__name__)
 
 
+def _influxdb() -> 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 with no InfluxDB behind it should
+    not have to carry the library to start.
+    """
+    try:
+        import influxdb_client
+    except ImportError:
+        raise RuntimeError(
+            "the influxdb node needs the server extra: pip install 'fluksio[server]'"
+        ) from None
+    return influxdb_client
+
+
 class InfluxDbNode(Node):
     """
     InfluxDB node for writing to and reading from InfluxDB.
@@ -290,7 +306,7 @@ class InfluxDbNode(Node):
         :returns: ``{"rows": [...], **echo}``.
         :rtype: dict
         """
-        from influxdb_client import InfluxDBClient
+        InfluxDBClient = _influxdb().InfluxDBClient
 
         flux = str(request["flux"])
         echo = {key: value for key, value in request.items() if key != "flux"}
@@ -338,8 +354,10 @@ class InfluxDbNode(Node):
             - Dict with "value" and "tags" keys: Value written with merged tags
         :type data: dict
         """
-        from influxdb_client import InfluxDBClient, Point, WritePrecision
-        from influxdb_client.client.write_api import SYNCHRONOUS
+        influxdb_client = _influxdb()
+        InfluxDBClient = influxdb_client.InfluxDBClient
+        Point, WritePrecision = influxdb_client.Point, influxdb_client.WritePrecision
+        SYNCHRONOUS = influxdb_client.client.write_api.SYNCHRONOUS
 
         try:
             with InfluxDBClient(
@@ -438,7 +456,7 @@ class InfluxDbNode(Node):
         :returns: Dict of port name to queried value.
         :rtype: dict
         """
-        from influxdb_client import InfluxDBClient
+        InfluxDBClient = _influxdb().InfluxDBClient
 
         results = {}