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.

server/Dockerfile.dev

@@ -1,11 +1,18 @@
-# Development Dockerfile with hot reload
+# Stage 1: Fetch platform server package from git
+FROM alpine:3.20 AS platform
+RUN apk add --no-cache git
+ARG PLATFORM_REPO=https://git.spatulaa.com/MayaTheShy/cc-platform-core.git
+ARG PLATFORM_BRANCH=master
+RUN git clone --depth 1 --branch "$PLATFORM_BRANCH" "$PLATFORM_REPO" /src \
+    && rm -rf /src/server/node_modules /src/.git
+
+# Stage 2: Development with hot reload
 FROM node:18-alpine
 
 WORKDIR /app
 
-# Copy platform server package from additional build context
-COPY --from=platform-server . /app/platform-server/
-RUN rm -rf /app/platform-server/node_modules
+# Copy platform server package from the git-clone stage
+COPY --from=platform /src/server /app/platform-server/
 
 # Install nodemon for hot reload
 RUN npm install -g nodemon