Bundle kaya-otel in the Docker image

The tavolo-app otel extra was never installed: requirements.txt was
locked without it and the Dockerfile installs the local distributions
with --no-deps, so any container with OTEL_ENABLED set died at import
with "kaya-otel is not installed".

Add the extra's full closure (kaya-otel plus its OpenTelemetry/protobuf
dependencies) to the pinned lock and refresh the comments that claimed
the extra had to be added separately.

server/Dockerfile

@@ -60,7 +60,8 @@ RUN --mount=type=cache,target=/root/.cache/pip \
 
 # The three sibling distributions, installed from the local tree (PEP 420
 # namespace packages sharing the `tavolo` namespace). --no-deps: every
-# third-party dependency is already pinned in requirements.txt.
+# third-party dependency — including the tavolo-app `otel` extra — is
+# already pinned in requirements.txt.
 COPY server/packages/ ./packages/
 RUN --mount=type=cache,target=/root/.cache/pip \
     /opt/venv/bin/pip install --no-deps ./packages/tavolo-platform ./packages/tavolo-scopone ./packages/tavolo-app