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/packages/harness/pyproject.toml

@@ -39,6 +39,9 @@ dependencies = [
 [project.optional-dependencies]
 postgres = [
     "asyncpg>=0.29",
+    "langgraph-checkpoint-postgres>=3.0.5",
+    "psycopg[binary]>=3.3.3",
+    "psycopg-pool>=3.3.0",
 ]
 
 [build-system]