fix(docker): clone platform from git instead of additional_contexts

The additional_contexts approach required cc-platform-core to exist on
the Docker host at a relative path. This fails on servers where the
repo layout differs. Instead, use a multi-stage build: stage 1 clones
cc-platform-core from Gitea (depth 1), stage 2 copies server/ into the
app and rewrites the file: path. Fully self-contained — no host deps.
Applied to both production and dev Dockerfiles.

docker-compose.yml

@@ -4,8 +4,6 @@ services:
     build:
       context: ./server
       dockerfile: Dockerfile
-      additional_contexts:
-        platform-server: ../cc-platform-core/server
     container_name: turtle-server
     ports:
       - "4200:3001"  # HTTP API + WebSocket (unified)