Wire IBuildDeckRepository into StoryService; GetDeckListAsync now looks
up the chapter's CharaId, fetches class-specific prebuilt/trial decks via
GetStoryDecksByClass, and loads all DefaultDecks for default_deck_list.
Class guard (1-8) leaves build/trial empty for non-class chapters, matching
prod behaviour.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
