feat: add alembic migrations and PostgreSQL CI support

- Add alembic dependency and initialize migration framework
- Configure async alembic env.py for SQLAlchemy 2.0
- Create initial migration for PostORM table
- Gate init_db() with SKIP_INIT_DB env var for CI/production
- Add PostgreSQL service to Woodpecker CI pipeline
- Create integration tests for migrations (TC-INT-001..002)
- Create integration tests for SQLAlchemyPostRepository (TC-INT-003..009)
- Add unit test for init_db skip behavior (TC-UNIT-901)
- All 176 tests pass, coverage 72.59%

tests/integration/conftest.py

@@ -0,0 +1,45 @@
+import asyncio
+import os
+from collections.abc import AsyncGenerator
+from typing import Generator
+
+import pytest
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+
+from app.infrastructure.config import settings
+from app.infrastructure.database.models import Base
+
+
+@pytest.fixture(scope="session")
+def event_loop() -> Generator[asyncio.AbstractEventLoop]:
+    loop = asyncio.get_event_loop_policy().new_event_loop()
+    yield loop
+    loop.close()
+
+
+@pytest.fixture(scope="session", autouse=True)
+async def setup_database() -> AsyncGenerator[None]:
+    db_url = os.environ.get("DB_URL", settings.database_url)
+    test_engine = create_async_engine(db_url)
+
+    async with test_engine.begin() as conn:
+        await conn.run_sync(Base.metadata.create_all)
+
+    yield
+
+    async with test_engine.begin() as conn:
+        await conn.run_sync(Base.metadata.drop_all)
+
+    await test_engine.dispose()
+
+
+@pytest.fixture
+async def db_session() -> AsyncGenerator[AsyncSession]:
+    db_url = os.environ.get("DB_URL", settings.database_url)
+    test_engine = create_async_engine(db_url)
+    session_factory = async_sessionmaker(test_engine, class_=AsyncSession, expire_on_commit=False)
+
+    async with session_factory() as session:
+        yield session
+
+    await test_engine.dispose()