feat: admin soft-delete and hard-delete for books

- Add `archived` bool to domain.BookMeta, pbBook, and Meilisearch bookDoc
- ArchiveBook / UnarchiveBook patch the PocketBase record; ListBooks filters
  archived=false so hidden books disappear from all public responses
- Meilisearch: add `archived` as a filterable attribute; Search and Catalogue
  always prepend `archived = false` to exclude archived books from results
- DeleteBook permanently removes the PocketBase record, all chapters_idx rows,
  MinIO chapter objects, cover image, and the Meilisearch document
- New BookAdminStore interface with ArchiveBook, UnarchiveBook, DeleteBook
- Admin HTTP endpoints: PATCH /api/admin/books/{slug}/archive|unarchive,
  DELETE /api/admin/books/{slug}
- PocketBase schema: archived field added to live pb.libnovel.cc and to
  pb-init-v3.sh (both create block and add_field migration)

backend/internal/bookstore/bookstore.go

@@ -216,6 +216,27 @@ type BookImporter interface {
 	Import(ctx context.Context, objectKey, fileType string) ([]Chapter, error)
 }
 
+// BookAdminStore covers admin-only operations for managing books in the catalogue.
+// All methods require admin authorisation at the HTTP handler level.
+type BookAdminStore interface {
+	// ArchiveBook sets archived=true on a book record, hiding it from all
+	// public search and catalogue responses. Returns ErrNotFound when the
+	// slug does not exist.
+	ArchiveBook(ctx context.Context, slug string) error
+
+	// UnarchiveBook clears archived on a book record, making it publicly
+	// visible again. Returns ErrNotFound when the slug does not exist.
+	UnarchiveBook(ctx context.Context, slug string) error
+
+	// DeleteBook permanently removes all data for a book:
+	//   - PocketBase books record
+	//   - All PocketBase chapters_idx records
+	//   - All MinIO chapter markdown objects ({slug}/chapter-*.md)
+	//   - MinIO cover image (covers/{slug}.jpg)
+	// The caller is responsible for also deleting the Meilisearch document.
+	DeleteBook(ctx context.Context, slug string) error
+}
+
 // ImportFileStore uploads raw import files to object storage.
 // Kept separate from BookImporter so the HTTP handler can upload the file
 // without a concrete type assertion, regardless of which Producer is wired.