feat(api): add GET /series/ongoing and GET /books/ongoing endpoints

Two new read routes for the home screen:
- /series/ongoing: partially read series sorted by last activity
- /books/ongoing: next unread book per ongoing series

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/api/src/openapi.rs

@@ -10,6 +10,8 @@ use utoipa::OpenApi;
         crate::reading_progress::update_reading_progress,
         crate::books::get_thumbnail,
         crate::books::list_series,
+        crate::books::ongoing_series,
+        crate::books::ongoing_books,
         crate::books::convert_book,
         crate::pages::get_page,
         crate::search::search_books,
@@ -49,6 +51,7 @@ use utoipa::OpenApi;
             crate::reading_progress::UpdateReadingProgressRequest,
             crate::books::SeriesItem,
             crate::books::SeriesPage,
+            crate::books::OngoingQuery,
             crate::pages::PageQuery,
             crate::search::SearchQuery,
             crate::search::SearchResponse,