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_preview_applies_manual_overrides.py

@@ -1,3 +1,4 @@
+from abogen.domain.enums import Language
 from abogen.webui.routes.utils import synthesize
 
 
@@ -45,7 +46,7 @@ def test_preview_applies_manual_override_before_normalization(monkeypatch):
         synthesize.generate_preview_audio(
             text="He said Unfu*k loudly.",
             voice_spec="M1",
-            language="en",
+            language=Language.EN_US,
             speed=1.0,
             use_gpu=False,
             tts_provider="supertonic",