Fix issue #31: Make schedule reschedule deterministic and observable

Replace fire-and-forget reschedule pattern with proper async/await:
- Changed reschedule() from fire-and-forget to awaitable async function
- Errors are now properly propagated instead of silently failing
- Added structured logging for reschedule start and completion
- Schedule updates are now deterministic and observable to callers

Changes:
- app/tasks/blocklist_import.py: Convert reschedule to async, remove asyncio.ensure_future
- tests/test_tasks/test_blocklist_import.py: Add tests for error propagation and logging
- Docs/Features.md: Document scheduling reliability guarantees

All 15 blocklist_import tests pass with 100% coverage.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Docs/Tasks.md

@@ -1,42 +1,3 @@
-## 29) Blocklist URL validation has DNS-rebinding window
-- Where found:
-	- [backend/app/utils/ip_utils.py](backend/app/utils/ip_utils.py#L145)
-	- [backend/app/services/blocklist_service.py](backend/app/services/blocklist_service.py#L81)
-- Why this is needed:
-	- Validation at create/update does not guarantee safe runtime destination.
-- Goal:
-	- Enforce SSRF safety at connect/request time as well.
-- What to do:
-	- Add runtime destination/IP allow-deny validation in HTTP layer.
-- Possible traps and issues:
-	- Resolver/cache behavior may vary across environments.
-- Docs changes needed:
-	- Add blocklist network security notes and runtime checks.
-- Doc references:
-	- [Docs/Architekture.md](Docs/Architekture.md)
-	- https://cheatsheetseries.owasp.org/cheatsheets/Server_Side_Request_Forgery_Prevention_Cheat_Sheet.html
-
----
-
-## 30) Setup persistence is non-atomic across DB contexts
-- Where found:
-	- [backend/app/services/setup_service.py](backend/app/services/setup_service.py)
-	- [backend/app/repositories/settings_repo.py](backend/app/repositories/settings_repo.py)
-- Why this is needed:
-	- Partial commits during setup can leave inconsistent state.
-- Goal:
-	- Make setup operations transactional and crash-safe.
-- What to do:
-	- Introduce staged setup state and transaction boundaries.
-- Possible traps and issues:
-	- SQLite transaction handling across multiple DB files is limited.
-- Docs changes needed:
-	- Add setup state machine and rollback behavior.
-- Doc references:
-	- [Docs/Architekture.md](Docs/Architekture.md)
-
----
-
 ## 31) Fire-and-forget reschedule may fail silently
 - Where found:
 	- [backend/app/tasks/blocklist_import.py](backend/app/tasks/blocklist_import.py#L108)