fix: add missing book API methods and types (searchBooks, getBook, getGenres, getAuthors)

docs/backend/search-discovery-spec.md

@@ -69,7 +69,7 @@ Enable users to search books within the library and discover new books via filte
 - **Search bar** at top: text input with debounced `onChange` → calls API with `q` param
 - **Filter row**: genre dropdown, author dropdown, reading status dropdown
   - Genre/Author dropdowns populated from `/api/books/genres/` and `/api/books/authors/`
-  - Reading status uses static enum values
+  - Reading status uses static enum values (`READING_STATUS_OPTIONS`)
 - **Results grid**: card layout showing cover, title, author, reading status badge
 - **Empty state**: "No books found" with clear message when results are empty
 - **Loading state**: spinner/skeleton while fetching
@@ -80,6 +80,15 @@ Enable users to search books within the library and discover new books via filte
 - Back button to return to library
 - Clean, mobile-responsive layout
 
+### API Client — `frontend/src/api/books.ts`
+
+| Method | Endpoint | Returns |
+|--------|----------|---------|
+| `searchBooks(params)` | `GET /api/books/` | `{count, results: BookListItem[]}` |
+| `getBook(id)` | `GET /api/books/{id}/` | `BookDetail` |
+| `getGenres()` | `GET /api/books/genres/` | `string[]` |
+| `getAuthors()` | `GET /api/books/authors/` | `string[]` |
+
 ## Routes (Frontend)
 | Path | Component | Auth |
 |------|-----------|------|