SIENTIAPDE-1973: fix(deps): make the gateway requirements installable from the cluster index

Workers install only from the cluster package index (install-entrypoint.sh:
pip install --index-url $PYPI_SERVER). The 2.0.0-rc1 deploy never became Ready
because two new requirements are not mirrored there:

- grpcio-tools: only needed to regenerate ingestor/proto/*_pb2*.py, which are
  committed. Moved to requirements-dev.txt (nothing imports grpc_tools).
- nats-py: needed at runtime; mirrored by library-distribution-controller#92.

Also declares protobuf>=7.35 explicitly: the committed stubs were generated with
protobuf 7.35.0 and validate the runtime version on import; until now it only
arrived transitively. The mirror carries 7.36.x.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

requirements.txt

@@ -8,4 +8,6 @@ prometheus_client
 pymongo
 nats-py>=2.6
 grpcio>=1.60
-grpcio-tools>=1.60
+# Runtime of the committed gRPC stubs: ingestor/proto/opc_gateway_pb2.py was
+# generated with protobuf 7.35.0 and validates the runtime version on import.
+protobuf>=7.35