chore: migrate to v3, Doppler secrets, clean up legacy code

- Remove all pre-v3 code: scraper, ui-v2, backend v1, ios v1+v2, legacy CI workflows
- Flatten v3/ contents to repo root
- Add Doppler secrets management (project=libnovel, config=prd)
- Add justfile with doppler run wrappers for all docker compose commands
- Strip hardcoded env fallbacks from docker-compose.yml
- Add minimal README.md
- Clean up .gitignore

backend/internal/runner/runner_test.go

@@ -146,7 +146,7 @@ func (s *stubNovelScraper) ScrapeMetadata(_ context.Context, _ string) (domain.B
 	return domain.BookMeta{Slug: "test-book", Title: "Test Book", SourceURL: "https://example.com/book/test-book"}, nil
 }
 
-func (s *stubNovelScraper) ScrapeChapterList(_ context.Context, _ string) ([]domain.ChapterRef, error) {
+func (s *stubNovelScraper) ScrapeChapterList(_ context.Context, _ string, _ int) ([]domain.ChapterRef, error) {
 	return s.chapters, nil
 }