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/tests/test_tasks/test_blocklist_import.py

@@ -176,7 +176,10 @@ class TestRunImport:
             # Must not raise — the task swallows unexpected errors.
             await _run_import(app)
 
-        mock_log.exception.assert_called_once_with("blocklist_import_unexpected_error")
+        mock_log.exception.assert_called_once()
+        call_args = mock_log.exception.call_args
+        assert call_args[0][0] == "blocklist_import_unexpected_error"
+        assert "correlation_id" in call_args[1]
 
 
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