refactor: unify Language enum across all layers

- EngineConfig.language: Language (was lang_code: str = 'a')
- Engine owns _KOKORO_LANG_MAP, engine_language(), supported_languages()
- Engine provides language_for_voice_id() for voice catalog
- Plugins/kokoro/__init__.py calls engine_language() internally
- create_pipeline(plugin_id, language=Language) — no kokoro codes
- pipeline_factory.py clean of kokoro-specific code
- Domain functions raise TypeError if non-enum passed
- WebUI api.py: _parse_language() helper at API boundary
- Voice catalog returns ISO codes (lang.value)
- Constants: LANGUAGE_DESCRIPTIONS keyed by Language enum
- All tests updated for Language enum
- 1414 tests pass

tests/test_conversion_request.py

@@ -37,13 +37,13 @@ class TestConversionRequestBasics:
         assert "merge_chapters_at_end" in defaults
 
     def test_split_pattern_computation(self):
-        pattern = get_split_pattern("a", "Disabled")
+        pattern = get_split_pattern(Language.EN_US, "Disabled")
         assert isinstance(pattern, str)
         assert len(pattern) > 0
 
     def test_split_pattern_varies_by_subtitle_mode(self):
-        pattern_disabled = get_split_pattern("a", "Disabled")
-        pattern_sentence = get_split_pattern("a", "Sentence")
+        pattern_disabled = get_split_pattern(Language.EN_US, "Disabled")
+        pattern_sentence = get_split_pattern(Language.EN_US, "Sentence")
         # Different modes should produce different patterns
         assert isinstance(pattern_disabled, str)
         assert isinstance(pattern_sentence, str)