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/cmd/backend/main.go

@@ -198,6 +198,7 @@ func run() error {
 			TextGen:           textGenClient,
 			BookWriter:        store,
 			AIJobStore:        store,
+			BookAdminStore:    store,
 			Log:               log,
 		},
 	)