Update observability docs and task utilities

- Add Observability.md documentation
- Standardize task logging with correlation_id support
- Add log_sanitizer utility for PII masking
- Update Tasks.md tracking
- Update geo_cache tasks and other task modules with correlation_id

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

backend/app/services/blocklist_service.py

@@ -391,7 +391,7 @@ def schedule_blocklist_job(
     settings: Settings,
     http_session: aiohttp.ClientSession,
     config: ScheduleConfig,
-    run_import_callback: Callable[[Settings, aiohttp.ClientSession], Awaitable[None]],
+    run_import_callback: Callable[..., Awaitable[None]],
 ) -> None:
     """Register or replace the scheduled blocklist import job."""