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/utils.py

@@ -4,7 +4,6 @@ from datetime import UTC, datetime, timedelta
 from pathlib import Path
 from typing import Any
 
-import emails  # type: ignore
 import jwt
 from jinja2 import Template
 from jwt.exceptions import InvalidTokenError
@@ -37,6 +36,12 @@ def send_email(
     html_content: str = "",
 ) -> None:
     assert settings.emails_enabled, "no provided configuration for email variables"
+    try:
+        import emails  # type: ignore
+    except ImportError:
+        raise RuntimeError(
+            "sending mail needs the server extra: pip install 'fluksio[server]'"
+        ) from None
     message = emails.Message(
         subject=subject,
         html=html_content,