feat: add alembic migrations, integration tests, and 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%

app/infrastructure/database/connection.py

@@ -78,7 +78,13 @@ async def init_db() -> None:
 
     Creates all tables defined in the metadata.
     Should be called on application startup.
+    Skipped in CI/production where alembic manages schema.
     """
+    import os
+
+    if os.environ.get("SKIP_INIT_DB", "").lower() in ("1", "true", "yes"):
+        return
+
     from app.infrastructure.database.models import Base
 
     async with engine.begin() as conn: