feat(docker): add UV_EXTRAS build arg for optional dependencies

Support installing optional dependency groups (e.g. postgres) at
Docker build time via UV_EXTRAS build arg:
  UV_EXTRAS=postgres docker compose build

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/Dockerfile

@@ -9,6 +9,9 @@ FROM python:3.12-slim-bookworm
 ARG NODE_MAJOR=22
 ARG APT_MIRROR
 ARG UV_INDEX_URL
+# Optional extras to install (e.g. "postgres" for PostgreSQL support)
+# Usage: docker build --build-arg UV_EXTRAS=postgres ...
+ARG UV_EXTRAS
 
 # Optionally override apt mirror for restricted networks (e.g. APT_MIRROR=mirrors.aliyun.com)
 RUN if [ -n "${APT_MIRROR}" ]; then \
@@ -42,8 +45,9 @@ WORKDIR /app
 COPY backend ./backend
 
 # Install dependencies with cache mount
+# When UV_EXTRAS is set (e.g. "postgres"), installs optional dependencies.
 RUN --mount=type=cache,target=/root/.cache/uv \
-    sh -c "cd backend && UV_INDEX_URL=${UV_INDEX_URL:-https://pypi.org/simple} uv sync"
+    sh -c "cd backend && UV_INDEX_URL=${UV_INDEX_URL:-https://pypi.org/simple} uv sync ${UV_EXTRAS:+--extra $UV_EXTRAS}"
 
 # Expose ports (gateway: 8001, langgraph: 2024)
 EXPOSE 8001 2024