feat: add PDF/EPUB import functionality

- Add ImportTask/ImportResult types to domain.go
- Add TypeImportBook to asynqqueue for task routing
- Add CreateImportTask to producer and storage layers
- Add ClaimNextImportTask/FinishImportTask to Consumer interfaces
- Add import task handling to runner (polling + Asynq handler)
- Add BookImporter interface to bookstore for PDF/EPUB parsing
- Add backend API endpoints: POST/GET /api/admin/import
- Add SvelteKit UI at /admin/import with task list
- Add nav link in admin layout

Note: PDF/EPUB parsing is a placeholder - needs external library integration.

backend/internal/bookstore/bookstore.go

@@ -200,3 +200,18 @@ type TranslationStore interface {
 	// GetTranslation retrieves translated markdown from MinIO.
 	GetTranslation(ctx context.Context, key string) (string, error)
 }
+
+// Chapter represents a single chapter extracted from PDF/EPUB.
+type Chapter struct {
+	Number   int    // 1-based chapter number
+	Title    string // chapter title (may be empty)
+	Content  string // plain text content
+}
+
+// BookImporter handles PDF/EPUB file parsing and chapter extraction.
+// Used by the runner to import books from uploaded files.
+type BookImporter interface {
+	// Import extracts chapters from a PDF or EPUB file stored in MinIO.
+	// Returns the extracted chapters or an error.
+	Import(ctx context.Context, objectKey, fileType string) ([]Chapter, error)
+}