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/asynqqueue/consumer.go

@@ -40,6 +40,10 @@ func (c *Consumer) FinishTranslationTask(ctx context.Context, id string, result
 	return c.pb.FinishTranslationTask(ctx, id, result)
 }
 
+func (c *Consumer) FinishImportTask(ctx context.Context, id string, result domain.ImportResult) error {
+	return c.pb.FinishImportTask(ctx, id, result)
+}
+
 func (c *Consumer) FailTask(ctx context.Context, id, errMsg string) error {
 	return c.pb.FailTask(ctx, id, errMsg)
 }
@@ -60,6 +64,12 @@ func (c *Consumer) ClaimNextTranslationTask(ctx context.Context, workerID string
 	return c.pb.ClaimNextTranslationTask(ctx, workerID)
 }
 
+// ClaimNextImportTask delegates to PocketBase because import tasks
+// are stored in PocketBase (not Redis/Asynq) and must still be polled directly.
+func (c *Consumer) ClaimNextImportTask(ctx context.Context, workerID string) (domain.ImportTask, bool, error) {
+	return c.pb.ClaimNextImportTask(ctx, workerID)
+}
+
 func (c *Consumer) HeartbeatTask(ctx context.Context, id string) error {
 	return c.pb.HeartbeatTask(ctx, id)
 }