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/meili/client.go

@@ -32,11 +32,15 @@ type Client interface {
 	// BookExists reports whether a book with the given slug is already in the
 	// index. Used by the catalogue refresh to skip re-indexing known books.
 	BookExists(ctx context.Context, slug string) bool
+	// DeleteBook removes a book document from the search index by slug.
+	DeleteBook(ctx context.Context, slug string) error
 	// Search returns up to limit books matching query.
+	// Archived books are always excluded.
 	Search(ctx context.Context, query string, limit int) ([]domain.BookMeta, error)
 	// Catalogue queries books with optional filters, sort, and pagination.
 	// Returns books, the total hit count for pagination, and a FacetResult
 	// with available genre and status values from the index.
+	// Archived books are always excluded.
 	Catalogue(ctx context.Context, q CatalogueQuery) ([]domain.BookMeta, int64, FacetResult, error)
 }
 
@@ -99,7 +103,7 @@ func Configure(host, apiKey string) error {
 		return fmt.Errorf("meili: update searchable attributes: %w", err)
 	}
 
-	filterable := []interface{}{"status", "genres"}
+	filterable := []interface{}{"status", "genres", "archived"}
 	if _, err := idx.UpdateFilterableAttributes(&filterable); err != nil {
 		return fmt.Errorf("meili: update filterable attributes: %w", err)
 	}
@@ -128,6 +132,9 @@ type bookDoc struct {
 	// MetaUpdated is the Unix timestamp (seconds) of the last PocketBase update.
 	// Used for sort=update ("recently updated" ordering).
 	MetaUpdated int64 `json:"meta_updated"`
+	// Archived is true when the book has been soft-deleted by an admin.
+	// Used as a filter to exclude archived books from all search results.
+	Archived bool `json:"archived"`
 }
 
 func toDoc(b domain.BookMeta) bookDoc {
@@ -144,6 +151,7 @@ func toDoc(b domain.BookMeta) bookDoc {
 		Rank:          b.Ranking,
 		Rating:        b.Rating,
 		MetaUpdated:   b.MetaUpdated,
+		Archived:      b.Archived,
 	}
 }
 
@@ -161,6 +169,7 @@ func fromDoc(d bookDoc) domain.BookMeta {
 		Ranking:       d.Rank,
 		Rating:        d.Rating,
 		MetaUpdated:   d.MetaUpdated,
+		Archived:      d.Archived,
 	}
 }
 
@@ -184,13 +193,24 @@ func (c *MeiliClient) BookExists(_ context.Context, slug string) bool {
 	return err == nil && doc.Slug != ""
 }
 
+// DeleteBook removes a book document from the index by slug.
+// The operation is fire-and-forget (Meilisearch processes tasks asynchronously).
+func (c *MeiliClient) DeleteBook(_ context.Context, slug string) error {
+	if _, err := c.idx.DeleteDocument(slug, nil); err != nil {
+		return fmt.Errorf("meili: delete book %q: %w", slug, err)
+	}
+	return nil
+}
+
 // Search returns books matching query, up to limit results.
+// Archived books are always excluded.
 func (c *MeiliClient) Search(_ context.Context, query string, limit int) ([]domain.BookMeta, error) {
 	if limit <= 0 {
 		limit = 20
 	}
 	res, err := c.idx.Search(query, &meilisearch.SearchRequest{
-		Limit: int64(limit),
+		Limit:  int64(limit),
+		Filter: "archived = false",
 	})
 	if err != nil {
 		return nil, fmt.Errorf("meili: search %q: %w", query, err)
@@ -231,17 +251,15 @@ func (c *MeiliClient) Catalogue(_ context.Context, q CatalogueQuery) ([]domain.B
 		Facets: []string{"genres", "status"},
 	}
 
-	// Build filter
-	var filters []string
+	// Build filter — always exclude archived books
+	filters := []string{"archived = false"}
 	if q.Genre != "" && q.Genre != "all" {
 		filters = append(filters, fmt.Sprintf("genres = %q", q.Genre))
 	}
 	if q.Status != "" && q.Status != "all" {
 		filters = append(filters, fmt.Sprintf("status = %q", q.Status))
 	}
-	if len(filters) > 0 {
-		req.Filter = strings.Join(filters, " AND ")
-	}
+	req.Filter = strings.Join(filters, " AND ")
 
 	// Map UI sort tokens to Meilisearch sort expressions.
 	switch q.Sort {
@@ -318,7 +336,8 @@ func sortStrings(s []string) {
 type NoopClient struct{}
 
 func (NoopClient) UpsertBook(_ context.Context, _ domain.BookMeta) error { return nil }
-func (NoopClient) BookExists(_ context.Context, _ string) bool           { return false }
+func (NoopClient) BookExists(_ context.Context, _ string) bool            { return false }
+func (NoopClient) DeleteBook(_ context.Context, _ string) error           { return nil }
 func (NoopClient) Search(_ context.Context, _ string, _ int) ([]domain.BookMeta, error) {
 	return nil, nil
 }