build(deps): add postgres-test optional extra for testcontainers

新增 postgres-test extra（harness + backend），引入 testcontainers[postgres]
4.14.2 用于 Stage 0 PR1 的 PG fixture。复用现有 postgres extra 的
asyncpg/psycopg/langgraph-checkpoint-postgres，pytest-asyncio 沿用 dev group。

Stage 0 PR1 T1.1.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/packages/harness/pyproject.toml

@@ -46,6 +46,10 @@ postgres = [
     "psycopg[binary]>=3.3.3",
     "psycopg-pool>=3.3.0",
 ]
+postgres-test = [
+    "deerflow-harness[postgres]",
+    "testcontainers[postgres]>=4.0",
+]
 pymupdf = ["pymupdf4llm>=0.0.17"]
 
 [build-system]