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/pyproject.toml

@@ -25,6 +25,7 @@ dependencies = [
 
 [project.optional-dependencies]
 postgres = ["deerflow-harness[postgres]"]
+postgres-test = ["deerflow-harness[postgres-test]"]
 
 [dependency-groups]
 dev = [