refactor: centralize integration config in domain layer, fix OutputFormat enum serialization

- Move stored_integration_config(), build_audiobookshelf_config() to domain/settings_core.py
- Remove legacy fallback from stored_integration_config() (only config[integrations])
- Add load_audiobookshelf_config() as combined entry point
- PostConversionHooks reads config directly via stored_integration_config()
- WebUI imports from domain/settings_core instead of webui/routes/utils/settings
- Remove duplicate _build_abs_config() from PostConversionHooks
- Remove dead audiobookshelf code from infrastructure/exporters.py
- Remove duplicate audiobookshelf functions from webui/service.py
- Fix OutputFormat enum serialization in output_layout_service.py and conversion_executor.py
  (f'{enum}' gave 'OutputFormat.WAV' instead of '.wav')
- Mock spaCy in test_returns_at_least_one_segment instead of loading real model
- Add 28 tests for PostConversionHooks and build_audiobookshelf_config

tests/test_domain_conversion_pipeline.py

@@ -251,13 +251,15 @@ class TestSpacyPreTtsSegmentation:
         assert len(segments) == 1
 
     def test_returns_at_least_one_segment(self):
+        from unittest.mock import patch
         from abogen.domain.conversion_pipeline import spacy_pre_tts_segmentation
         from abogen.domain.enums import Language
 
-        segments, split = spacy_pre_tts_segmentation(
-            "",
-            Language.FR,
-            "Sentence",
-            use_spacy_segmentation=True,
-        )
+        with patch("abogen.spacy_utils.segment_sentences", return_value=None):
+            segments, split = spacy_pre_tts_segmentation(
+                "",
+                Language.FR,
+                "Sentence",
+                use_spacy_segmentation=True,
+            )
         assert len(segments) >= 1