Quality cleanup refactoring

Made-with: Cursor

_docs/04_refactoring/01-quality-cleanup/test_sync/new_tests.md

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+# New Tests
+
+No new tests added. All new internal code paths are exercised by existing e2e tests:
+
+| New Code | Covered By |
+|----------|------------|
+| `_UnlockStateHolder` class | test_unlock_status_idle, test_unlock_missing_archive, test_unlock_concurrent_returns_current_state |
+| Double-checked locking in `get_api_client()` | All endpoint tests (each calls get_api_client) |
+| CDN upload exception raise | test_upload_resource, test_upload_download_roundtrip |
+| Library PKCS7 unpadder (security.pyx) | test_upload_download_roundtrip (encrypt→decrypt roundtrip) |
+| Library PKCS7 unpadder (binary_split.py) | test_unlock_with_corrupt_archive (error path) |
+| `upload_file` exception propagation | test_upload_resource |
+| `LOG_DIR` env var | Indirect (service starts successfully with default) |

_docs/04_refactoring/01-quality-cleanup/test_sync/obsolete_tests.md

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+# Obsolete Tests
+
+None identified. Removed methods (get_user, list_files, check_resource, upload_to_cdn, download_from_cdn) and constants had no existing tests — they were orphans with no callers or test references.

_docs/04_refactoring/01-quality-cleanup/test_sync/updated_tests.md

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+# Updated Tests
+
+No test updates required. All 18 e2e tests pass without modification — the refactoring preserved all public API contracts (endpoints, request/response formats, error codes).