refactor(webui): rename preview.py to synthesize.py and remove dead code

- Rename abogen/webui/routes/utils/preview.py → synthesize.py
  The file contains the core TTS synthesis pipeline (generate_preview_audio,
  synthesize_preview), not just preview logic. Name now matches responsibility.
- Remove dead code from voice.py: get_preview_pipeline(), synthesize_audio_from_normalized(),
  _preview_pipeline_lock, _preview_pipelines, and unused imports (threading, numpy,
  create_pipeline, get_new_voice, _select_device, _to_float32, SAMPLE_RATE, SPLIT_PATTERN).
  These were never called — identical logic lives in synthesize.py.
- Update imports in api.py, voices.py, and test_preview_applies_manual_overrides.py
- 7 new tests in test_synthesize_module.py enforce file naming and import rules
- 7 tests in test_domain_imports.py updated for renamed module

tests/test_preview_applies_manual_overrides.py

@@ -1,11 +1,11 @@
-from abogen.webui.routes.utils import preview
+from abogen.webui.routes.utils import synthesize
 
 
 def test_preview_applies_manual_override_before_normalization(monkeypatch):
     # Don't run real TTS/normalization; just exercise the override stage by
     # forcing provider=kokoro and then stubbing normalize_for_pipeline.
 
-    monkeypatch.setattr(preview, "get_preview_pipeline", lambda language, device: None)
+    monkeypatch.setattr(synthesize, "get_preview_pipeline", lambda language, device: None)
 
     # Stub normalize_for_pipeline to be identity; we only care that overrides run.
     class _Norm:
@@ -42,7 +42,7 @@ def test_preview_applies_manual_override_before_normalization(monkeypatch):
     monkeypatch.setattr(utils, "create_pipeline", _mock_create_pipeline)
 
     try:
-        preview.generate_preview_audio(
+        synthesize.generate_preview_audio(
             text="He said Unfu*k loudly.",
             voice_spec="M1",
             language="en",