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/domain/domain.go

@@ -24,6 +24,9 @@ type BookMeta struct {
 	// updated in PocketBase. Populated on read; not sent on write (PocketBase
 	// manages its own updated field).
 	MetaUpdated int64 `json:"meta_updated,omitempty"`
+	// Archived is true when the book has been soft-deleted by an admin.
+	// Archived books are excluded from all public search and catalogue responses.
+	Archived bool `json:"archived,omitempty"`
 }
 
 // CatalogueEntry is a lightweight book reference returned by catalogue pages.