feat(dl): gate s_lang/a_lang miss behind --best-available

Missing requested subtitle and audio languages now warn and continue when --best-available is set instead of hard-exiting. Without the flag, missing languages still produce an error and exit, matching the prior strict behavior. Audio missing-lang detection is now symmetric with subtitles.

- add find_missing_langs helper in core/utilities for reuse between s_lang and a_lang paths (skips all/best/orig sentinels)
- refactor dl.py s_lang/a_lang checks to share the helper
- add tests/lang_selection covering match primitives, helper output, and tricky langcodes corners (zh-Hans/zh-Hant/zh-CN/zh-TW/zh-HK, cmn/yue, fil/tl/tgl)
- clean up unused-var ruff F841 in tests/remote/unit/

tests/remote/unit/test_errors.py

@@ -6,13 +6,8 @@ import json
 
 import pytest
 
-from unshackle.core.api.errors import (
-    APIError,
-    APIErrorCode,
-    build_error_response,
-    categorize_exception,
-    handle_api_exception,
-)
+from unshackle.core.api.errors import (APIError, APIErrorCode, build_error_response, categorize_exception,
+                                       handle_api_exception)
 
 pytestmark = pytest.mark.unit