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/main.py

@@ -4,7 +4,6 @@ import logging
 from collections.abc import AsyncIterator
 from contextlib import AbstractAsyncContextManager, asynccontextmanager
 
-import sentry_sdk
 from fastapi import FastAPI, Request
 from fastapi.concurrency import run_in_threadpool
 from fastapi.responses import JSONResponse
@@ -51,6 +50,10 @@ def custom_generate_unique_id(route: APIRoute) -> str:
 
 
 if settings.SENTRY_DSN and settings.ENVIRONMENT != "local":
+    # Imported here rather than at the top: it is a `fluksio[server]` extra, so
+    # a pip install without one has no sentry to import — and no DSN either.
+    import sentry_sdk
+
     # `enable_tracing` was removed in sentry-sdk 2.x; this is what it meant.
     sentry_sdk.init(dsn=str(settings.SENTRY_DSN), traces_sample_rate=1.0)