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_service.py

@@ -7,8 +7,8 @@ from abogen.webui.service import (
     JobStatus,
     build_service,
     _JOB_LOGGER,
-    build_audiobookshelf_metadata,
 )
+from abogen.domain.metadata_helpers import build_audiobookshelf_metadata
 
 
 def test_service_processes_job(tmp_path):
@@ -233,7 +233,7 @@ def test_audiobookshelf_metadata_uses_book_number(tmp_path):
         },
     )
 
-    metadata = build_audiobookshelf_metadata(job)
+    metadata = build_audiobookshelf_metadata(job.metadata_tags)
 
     assert metadata["seriesName"] == "Example Saga"
     assert metadata["seriesSequence"] == "7"
@@ -264,7 +264,7 @@ def test_audiobookshelf_metadata_normalizes_sequence_value(tmp_path):
         },
     )
 
-    metadata = build_audiobookshelf_metadata(job)
+    metadata = build_audiobookshelf_metadata(job.metadata_tags)
 
     assert metadata["seriesName"] == "Example Saga"
     assert metadata["seriesSequence"] == "7"
@@ -295,6 +295,6 @@ def test_audiobookshelf_metadata_allows_decimal_sequence(tmp_path):
         },
     )
 
-    metadata = build_audiobookshelf_metadata(job)
+    metadata = build_audiobookshelf_metadata(job.metadata_tags)
 
     assert metadata["seriesSequence"] == "4.5"