test(fixtures): add testcontainers Postgres fixture for Stage 0

新建 backend/tests/fixtures/postgres.py，提供两层 fixture：
  - postgres_container (session 级)：用 PostgresContainer 起 postgres:16-alpine
  - postgres_url (function 级)：每 test 一个 ephemeral DB，teardown 时
    pg_terminate_backend 清掉残连后 DROP DATABASE

为什么 per-DB 而不是 per-schema：asyncpg + SQLAlchemy 不通过 URL 传 search_path，
per-DB 一次 ~50ms 开销可接受，让 test 代码不感知 schema。

Docker 不可用时 fixture 自动 pytest.skip 而非 error，dev 环境无 Docker
仍能跑其余 3086 个测试。

注册 pytest mark `postgres`、把 tests/ 加 sys.path 让 pytest_plugins
按 `fixtures.postgres` 路径解析（不加 tests/__init__.py 避免干扰
现有 pytest 发现行为）。

Stage 0 PR1 T1.3.

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

backend/pyproject.toml

@@ -38,6 +38,7 @@ dev = [
 [tool.pytest.ini_options]
 markers = [
     "no_auto_user: disable the conftest autouse contextvar fixture for this test",
+    "postgres: requires a Postgres testcontainer (Docker daemon); skipped if unavailable",
 ]
 
 [tool.uv]

backend/tests/conftest.py

@@ -15,6 +15,13 @@ import pytest
 # Make 'app' and 'deerflow' importable from any working directory
 sys.path.insert(0, str(Path(__file__).parent.parent))
 sys.path.insert(0, str(Path(__file__).resolve().parents[2] / "scripts"))
+# Make 'fixtures.*' importable as plugin modules from this conftest.
+sys.path.insert(0, str(Path(__file__).parent))
+
+# Register fixture plugin modules so tests can request fixtures by name
+# without ad-hoc imports. ``fixtures.postgres`` provides
+# ``postgres_container`` (session-scoped) and ``postgres_url`` (per-test).
+pytest_plugins = ["fixtures.postgres"]
 
 # Break the circular import chain that exists in production code:
 #   deerflow.subagents.__init__

backend/tests/fixtures/postgres.py

@@ -0,0 +1,80 @@
+"""Postgres testcontainer fixtures for Stage 0 PR1.
+
+Provides per-test ephemeral database isolation atop a single session-scoped
+container. Tests marked ``@pytest.mark.postgres`` request the ``postgres_url``
+fixture, which yields an asyncpg connection URL pointing at a freshly-created
+database. The database is force-dropped after the test (any leaked connections
+get pg_terminate_backend'd first).
+
+Why per-database rather than per-schema:
+  asyncpg (the SQLAlchemy async driver we use) doesn't honor URL-embedded
+  search_path the way psycopg does. Per-database isolation is one extra
+  CREATE/DROP per test (~50ms), but lets test app code use its full schema
+  unchanged.
+"""
+
+from __future__ import annotations
+
+import secrets
+from collections.abc import Iterator
+
+import pytest
+
+
+@pytest.fixture(scope="session")
+def postgres_container():
+    """Session-scoped Postgres 16 container shared across all postgres tests.
+
+    Started once per pytest session. Subsequent tests piggy-back on the same
+    container; each gets its own database via the ``postgres_url`` fixture.
+
+    Skipped (and the test marked skip) if Docker is unavailable on the host —
+    testcontainers raises ``DockerException`` when it can't reach the daemon.
+    """
+    try:
+        from testcontainers.postgres import PostgresContainer
+    except ImportError as exc:  # pragma: no cover - install boundary
+        pytest.skip(f"testcontainers[postgres] not installed: {exc}")
+
+    try:
+        with PostgresContainer("postgres:16-alpine") as pg:
+            yield pg
+    except Exception as exc:  # pragma: no cover - environment-dependent
+        # DockerException, ConnectionError, etc. — surface a skip rather than
+        # an error so devs without Docker can still run the rest of the suite.
+        pytest.skip(f"could not start Postgres container ({exc})")
+
+
+@pytest.fixture
+def postgres_url(postgres_container) -> Iterator[str]:
+    """Per-test ephemeral database URL (asyncpg dialect).
+
+    Each invocation creates a unique database on the shared container and
+    yields its URL. Teardown force-drops the database, terminating any
+    backend connections the test forgot to close.
+    """
+    import psycopg
+    from psycopg import sql
+
+    db_name = f"test_{secrets.token_hex(8)}"
+    raw = postgres_container.get_connection_url()  # postgresql+psycopg2://...
+    # Strip the SQLAlchemy dialect prefix so plain psycopg can connect.
+    base = raw.replace("postgresql+psycopg2://", "postgresql://")
+    parent_url = base.rsplit("/", 1)[0] + "/postgres"
+
+    # CREATE DATABASE must run outside a transaction; psycopg autocommit=True.
+    with psycopg.connect(parent_url, autocommit=True) as conn:
+        conn.execute(sql.SQL("CREATE DATABASE {}").format(sql.Identifier(db_name)))
+
+    asyncpg_url = raw.replace("postgresql+psycopg2://", "postgresql+asyncpg://").rsplit("/", 1)[0] + f"/{db_name}"
+
+    try:
+        yield asyncpg_url
+    finally:
+        with psycopg.connect(parent_url, autocommit=True) as conn:
+            # Kick any leaked connections so DROP DATABASE doesn't block.
+            conn.execute(
+                "SELECT pg_terminate_backend(pid) FROM pg_stat_activity WHERE datname = %s AND pid <> pg_backend_pid()",
+                (db_name,),
+            )
+            conn.execute(sql.SQL("DROP DATABASE IF EXISTS {}").format(sql.Identifier(db_name)))