Add sync db sessionmaker for Celery tasks

[szyszkapiotr]

Related issue: #121

In my previous MR (#108) we have migrated from sync to async SQLAlchemy sessions. That fits the API layer very well (FastAPI is async-first), but async sessions may cause problems in a purely synchronous context — namely inside Celery tasks. For that specific case I propose using sync db sessions, since running anything async inside Celery is famously painful.

This MR adds a sync db sessionmaker and lays out how to handle database operations inside Celery tasks. Rule of thumb:

• use async db sessions in the API layer;
• use sync db sessions inside Celery tasks. If that's not possible (e.g. you need to reuse a service exposing async methods), wrap the call with asyncio.run.

In this MR:

• added a sync db sessionmaker in app/database.py;
• updated the docs (AGENTS.md) to describe the approach above;
• added example tasks showing how to perform db operations (app/integrations/celery/tasks/dummy_task.py).