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/Dockerfile

@@ -31,7 +31,7 @@ ENV NODE_VENV=managed
 RUN --mount=type=cache,target=/root/.cache/uv \
     --mount=type=bind,source=uv.lock,target=uv.lock \
     --mount=type=bind,source=pyproject.toml,target=pyproject.toml \
-    uv sync --frozen --no-install-workspace --package fluksio
+    uv sync --frozen --no-install-workspace --package fluksio --extra server
 
 COPY ./backend/scripts /app/backend/scripts
 
@@ -52,7 +52,7 @@ COPY ./worker /app/worker
 RUN --mount=type=cache,target=/root/.cache/uv \
     --mount=type=bind,source=uv.lock,target=uv.lock \
     --mount=type=bind,source=pyproject.toml,target=pyproject.toml \
-    uv sync --frozen --package fluksio
+    uv sync --frozen --package fluksio --extra server
 
 # Connectors are ordinary installed packages found through the
 # `fluksio.node_types` entry point. `make connectors` builds them into here;