refactor: group ConversionRequest fields into config objects

Domain config types (domain/config_types.py):
- PronunciationConfig: pronunciation/heteronym/normalization overrides
- SubtitleConfig: mode, format, max_words
- CoverConfig: path, mime

Domain functions now accept config objects:
- build_tts_context(subtitle=, pronunciation=) instead of 9 individual params
- make_subtitle_writer(subtitle=) instead of 3 params
- process_and_write_subtitles(subtitle=) instead of 2 params
- embed_m4b_metadata(cover=) instead of 2 params
- build_epub3_package(cover=) instead of 2 params

ConversionRequest: 18 flat fields + 8 config objects
Application/config.py re-exports domain types
All tests updated to new API

tests/test_conversion_planner.py

@@ -125,12 +125,12 @@ class TestBuildConversionPlan:
 
     def test_output_layout(self):
         """Output layout is resolved from request."""
+        from abogen.application.conversion_config import SaveConfig
         with tempfile.TemporaryDirectory() as tmpdir:
             req = ConversionRequest(
                 direct_text="Hello",
                 voice="M1",
-                save_mode="custom_folder",
-                output_folder=Path(tmpdir),
+                save=SaveConfig(mode="custom_folder", output_folder=Path(tmpdir)),
             )
             plan = build_conversion_plan(req)