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_synthesize_module.py

@@ -0,0 +1,70 @@
+"""Tests that preview/synthesis module is correctly named and importable."""
+import pathlib
+
+
+def _read_source(module_file: str) -> str:
+    return pathlib.Path(module_file).read_text(encoding="utf-8")
+
+
+def test_preview_file_renamed_to_synthesize():
+    """preview.py must be renamed to synthesize.py."""
+    import abogen.webui.routes.utils.synthesize as synthesize_mod
+
+    synthesize_path = pathlib.Path(synthesize_mod.__file__)
+    assert synthesize_path.name == "synthesize.py"
+    assert synthesize_path.exists()
+    assert not synthesize_path.with_name("preview.py").exists()
+
+
+def test_synthesize_module_has_generate_preview_audio():
+    """synthesize.py must export generate_preview_audio."""
+    from abogen.webui.routes.utils.synthesize import generate_preview_audio
+
+    assert callable(generate_preview_audio)
+
+
+def test_synthesize_module_has_synthesize_preview():
+    """synthesize.py must export synthesize_preview."""
+    from abogen.webui.routes.utils.synthesize import synthesize_preview
+
+    assert callable(synthesize_preview)
+
+
+def test_synthesize_module_has_get_preview_pipeline():
+    """synthesize.py must export get_preview_pipeline."""
+    from abogen.webui.routes.utils.synthesize import get_preview_pipeline
+
+    assert callable(get_preview_pipeline)
+
+
+def test_api_imports_from_synthesize():
+    """api.py must import from synthesize, not preview."""
+    import abogen.webui.routes.api as api_mod
+
+    source = _read_source(api_mod.__file__)
+    assert "from abogen.webui.routes.utils.synthesize import" in source
+    assert "from abogen.webui.routes.utils.preview import" not in source
+
+
+def test_voices_imports_from_synthesize():
+    """voices.py must import from synthesize, not preview."""
+    import abogen.webui.routes.voices as voices_mod
+
+    source = _read_source(voices_mod.__file__)
+    assert "from abogen.webui.routes.utils.synthesize import" in source
+    assert "from abogen.webui.routes.utils.preview import" not in source
+
+
+def test_no_module_imports_preview():
+    """No module should import from the old preview path."""
+    import glob
+    import os
+
+    project_root = pathlib.Path(__file__).parent.parent
+    py_files = glob.glob(str(project_root / "abogen" / "**" / "*.py"), recursive=True)
+
+    for py_file in py_files:
+        content = pathlib.Path(py_file).read_text(encoding="utf-8")
+        assert "from abogen.webui.routes.utils.preview import" not in content, (
+            f"{py_file} still imports from preview"
+        )