feat(test): add PostgreSQL migration slow integration tests (G-003)

- Add tests/integration/persistence/test_migrations_postgres.py
- All tests marked with @pytest.mark.slow
- Tests skip when TEST_POSTGRES_URL is not set (no local PostgreSQL)
- Database isolation via clean_tables and clean_alembic_version fixtures
- Update CI workflow to use pytest instead of inline Python script
- Remove TODO(G-003) comment
- Update tests/README.md with PostgreSQL test documentation

Covered scenarios:
- Baseline stamp sets revision
- Upgrade from baseline to head
- Upgrade idempotent
- Get current on unstamped DB returns None

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/README.md

@@ -244,8 +244,27 @@ SQLite migration tests can be run locally without any external dependencies:
 uv run pytest tests/integration/persistence/test_migrations.py -q --tb=short
 ```
 
-CI workflow `.github/workflows/test-migrations.yml` runs SQLite tests using pytest.
-PostgreSQL migration tests still use inline Python script (will be migrated to pytest in G-003).
+PostgreSQL migration tests require an external PostgreSQL database:
+
+```bash
+# PostgreSQL migration tests (requires PostgreSQL service)
+# Tests are marked as slow and skipped if TEST_POSTGRES_URL is not set
+TEST_POSTGRES_URL=postgresql+asyncpg://user:pass@localhost:5432/test_db \
+    uv run pytest tests/integration/persistence/test_migrations_postgres.py -q --tb=short
+
+# Or skip by default (no PostgreSQL available)
+uv run pytest tests/integration/persistence/test_migrations_postgres.py -q --tb=short
+# Output: SKIPPED (TEST_POSTGRES_URL not set)
+```
+
+Note: PostgreSQL tests are **not** included in fast integration gate because they:
+- Require external PostgreSQL service
+- Are marked with `@pytest.mark.slow`
+- Need `TEST_POSTGRES_URL` environment variable
+
+CI workflow `.github/workflows/test-migrations.yml` runs:
+- SQLite tests in `test-migrations-sqlite` job (fast, no external services)
+- PostgreSQL tests in `test-migrations-postgres` job (uses PostgreSQL service container)
 
 ### Running pipeline integration tests locally
 
@@ -367,8 +386,8 @@ Check that you're mocking at the right level and using `AsyncMock` for async fun
 ## Future Enhancements
 
 - [x] Add integration tests for database migrations (SQLite)
-- [ ] Add PostgreSQL migration integration tests (G-003)
-- [ ] Add integration tests for full pipeline execution
+- [x] Add PostgreSQL migration integration tests (G-003)
+- [x] Add integration tests for full pipeline execution
 - [x] Add API smoke integration tests
 - [ ] Add E2E tests
 - [ ] Add performance benchmarks

tests/integration/persistence/test_migrations_postgres.py

@@ -0,0 +1,217 @@
+"""
+PostgreSQL migration integration tests.
+
+Tests real Alembic migration behavior using PostgreSQL database.
+Marked as slow - requires external PostgreSQL service.
+
+Run locally (requires PostgreSQL):
+    TEST_POSTGRES_URL=postgresql+asyncpg://user:pass@localhost:5432/test_db \
+        uv run pytest tests/integration/persistence/test_migrations_postgres.py -q
+
+CI runs automatically with PostgreSQL service container.
+"""
+
+from __future__ import annotations
+
+import os
+import pytest
+from sqlalchemy.ext.asyncio import create_async_engine
+from sqlalchemy import text
+
+from langbot.pkg.entity.persistence.base import Base
+from langbot.pkg.persistence.alembic_runner import (
+    run_alembic_upgrade,
+    run_alembic_stamp,
+    get_alembic_current,
+)
+
+
+pytestmark = [pytest.mark.integration, pytest.mark.slow]
+
+
+@pytest.fixture
+def postgres_url():
+    """Get PostgreSQL URL from environment."""
+    url = os.environ.get('TEST_POSTGRES_URL')
+    if not url:
+        pytest.skip("TEST_POSTGRES_URL not set")
+    return url
+
+
+@pytest.fixture
+async def postgres_engine(postgres_url):
+    """Create async PostgreSQL engine."""
+    engine = create_async_engine(postgres_url, isolation_level="AUTOCOMMIT")
+    yield engine
+    await engine.dispose()
+
+
+@pytest.fixture
+async def clean_tables(postgres_engine):
+    """Drop all tables before and after each test for isolation."""
+    # Drop all tables before test
+    async with postgres_engine.begin() as conn:
+        await conn.run_sync(Base.metadata.drop_all)
+
+    yield
+
+    # Drop all tables after test
+    async with postgres_engine.begin() as conn:
+        await conn.run_sync(Base.metadata.drop_all)
+
+
+@pytest.fixture
+async def clean_alembic_version(postgres_engine):
+    """Drop alembic_version table before and after each test."""
+    async with postgres_engine.begin() as conn:
+        # Drop alembic_version table if exists
+        try:
+            await conn.execute(text("DROP TABLE IF EXISTS alembic_version"))
+        except Exception:
+            pass
+
+    yield
+
+    async with postgres_engine.begin() as conn:
+        try:
+            await conn.execute(text("DROP TABLE IF EXISTS alembic_version"))
+        except Exception:
+            pass
+
+
+class TestPostgreSQLMigrationBaseline:
+    """Tests for baseline stamp workflow on PostgreSQL."""
+
+    @pytest.mark.asyncio
+    async def test_postgres_baseline_stamp_sets_revision(
+        self, postgres_engine, clean_tables, clean_alembic_version
+    ):
+        """
+        Stamp baseline on existing tables sets correct revision.
+
+        Workflow:
+        1. Create tables via Base.metadata.create_all
+        2. Stamp with '0001_baseline'
+        3. Verify current revision is '0001_baseline'
+        """
+        # Create all tables (simulates existing DB created by ORM)
+        async with postgres_engine.begin() as conn:
+            await conn.run_sync(Base.metadata.create_all)
+
+        # Stamp baseline
+        await run_alembic_stamp(postgres_engine, '0001_baseline')
+
+        # Verify revision
+        rev = await get_alembic_current(postgres_engine)
+        assert rev == '0001_baseline', f"Expected '0001_baseline', got {rev}"
+
+    @pytest.mark.asyncio
+    async def test_postgres_baseline_stamp_on_empty_db(
+        self, postgres_engine, clean_tables, clean_alembic_version
+    ):
+        """
+        Stamp on empty database (no tables) still sets revision.
+
+        This is an edge case - stamping without tables.
+        """
+        # Don't create tables - stamp directly
+        await run_alembic_stamp(postgres_engine, '0001_baseline')
+
+        rev = await get_alembic_current(postgres_engine)
+        assert rev == '0001_baseline'
+
+
+class TestPostgreSQLMigrationUpgrade:
+    """Tests for upgrade to head workflow on PostgreSQL."""
+
+    @pytest.mark.asyncio
+    async def test_postgres_upgrade_from_baseline_to_head(
+        self, postgres_engine, clean_tables, clean_alembic_version
+    ):
+        """
+        Upgrade from baseline to head applies all migrations.
+
+        Workflow:
+        1. Create tables
+        2. Stamp baseline
+        3. Upgrade to head
+        4. Verify current revision is head
+        """
+        # Create tables
+        async with postgres_engine.begin() as conn:
+            await conn.run_sync(Base.metadata.create_all)
+
+        # Stamp baseline
+        await run_alembic_stamp(postgres_engine, '0001_baseline')
+
+        # Upgrade to head
+        await run_alembic_upgrade(postgres_engine, 'head')
+
+        # Verify revision
+        rev = await get_alembic_current(postgres_engine)
+        assert rev is not None, "Expected a revision after upgrade"
+        # Head should be the latest migration (0003 for current state)
+        assert rev.startswith('0003'), f"Expected head to be 0003_*, got {rev}"
+
+    @pytest.mark.asyncio
+    async def test_postgres_upgrade_idempotent(
+        self, postgres_engine, clean_tables, clean_alembic_version
+    ):
+        """
+        Running upgrade to head multiple times is idempotent.
+
+        Workflow:
+        1. Upgrade to head
+        2. Get revision
+        3. Upgrade to head again
+        4. Verify same revision
+        """
+        # Create tables
+        async with postgres_engine.begin() as conn:
+            await conn.run_sync(Base.metadata.create_all)
+
+        # Stamp and upgrade
+        await run_alembic_stamp(postgres_engine, '0001_baseline')
+        await run_alembic_upgrade(postgres_engine, 'head')
+
+        rev1 = await get_alembic_current(postgres_engine)
+
+        # Upgrade again - should be idempotent
+        await run_alembic_upgrade(postgres_engine, 'head')
+
+        rev2 = await get_alembic_current(postgres_engine)
+        assert rev2 == rev1, f"Expected {rev1}, got {rev2}"
+
+
+class TestPostgreSQLMigrationGetCurrent:
+    """Tests for get_alembic_current behavior on PostgreSQL."""
+
+    @pytest.mark.asyncio
+    async def test_postgres_get_current_on_unstamped_db_returns_none(
+        self, postgres_engine, clean_tables, clean_alembic_version
+    ):
+        """
+        get_alembic_current returns None for unstamped database.
+        """
+        # Create tables but don't stamp
+        async with postgres_engine.begin() as conn:
+            await conn.run_sync(Base.metadata.create_all)
+
+        # No stamp - should return None
+        rev = await get_alembic_current(postgres_engine)
+        assert rev is None, f"Expected None for unstamped DB, got {rev}"
+
+    @pytest.mark.asyncio
+    async def test_postgres_get_current_after_stamp_returns_revision(
+        self, postgres_engine, clean_tables, clean_alembic_version
+    ):
+        """
+        get_alembic_current returns correct revision after stamp.
+        """
+        async with postgres_engine.begin() as conn:
+            await conn.run_sync(Base.metadata.create_all)
+
+        await run_alembic_stamp(postgres_engine, '0001_baseline')
+
+        rev = await get_alembic_current(postgres_engine)
+        assert rev == '0001_baseline'