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/scraper/scraper.go

@@ -20,8 +20,10 @@ type MetadataProvider interface {
 }
 
 // ChapterListProvider can enumerate all chapters of a book.
+// upTo > 0 stops pagination once at least upTo chapter numbers have been
+// collected (early-exit optimisation for range scrapes). upTo == 0 fetches all pages.
 type ChapterListProvider interface {
-	ScrapeChapterList(ctx context.Context, bookURL string) ([]domain.ChapterRef, error)
+	ScrapeChapterList(ctx context.Context, bookURL string, upTo int) ([]domain.ChapterRef, error)
 }
 
 // ChapterTextProvider can extract the readable text from a single chapter page.