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/tests/api/routes/test_runs.py

@@ -433,9 +433,7 @@ def test_a_run_records_the_inputs_it_actually_starts_from():
         name="study",
         mode="batch",
         inputs=[
-            FlowInput(
-                spec=MessageSpec(name="lr", dtype=DType.FLOAT), initial=0.01
-            ),
+            FlowInput(spec=MessageSpec(name="lr", dtype=DType.FLOAT), initial=0.01),
             FlowInput(spec=MessageSpec(name="epochs", dtype=DType.INT)),
         ],
     )