Standardize gRPC ports to 50051 in Terraform config

[rowan-stein]

User Request

Standardize all gRPC services to use port 50051 (de facto gRPC convention).

Changes in stacks/platform/main.tf

docker-runner values

1. env DOCKER_RUNNER_PORT — change from "7071" to "50051"
2. service.ports[grpc].port — change from 7071 to 50051
3. containerPorts[grpc].containerPort — change from 7071 to 50051

platform-server values

4. env DOCKER_RUNNER_GRPC_PORT — change from "7071" to "50051"
5. env DOCKER_RUNNER_PORT — change from "7071" to "50051"

[rowan-stein]

Completed — PR #69 merged.