Document task DB access and unify background task DB handling

Docs/Architekture.md

@@ -687,6 +687,15 @@ APScheduler 4.x (async mode) manages recurring background tasks.
 
 ---
 
+## 7.1 Background Tasks and Database Access
+
+- APScheduler jobs run outside FastAPI request/response scope and therefore cannot rely on ``Depends(get_db)``.
+- Background tasks must open their own application database connection via ``app.db.open_db`` and close it when the work completes.
+- Use a shared task helper (``app.tasks.db.task_db``) so every task follows the same async context manager pattern and avoids connection leaks.
+- This pattern is intentional: task code is structurally separate from request-handling dependencies and should not attempt to reuse request-scoped DB connections.
+
+---
+
 ## 8. API Design
 
 ### 8.1 Conventions