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.

ui/src/routes/api/admin/import/+server.ts

@@ -0,0 +1,38 @@
+import { json, error } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { backendFetch } from '$lib/server/scraper';
+
+/**
+ * GET /api/admin/import
+ * List all import tasks.
+ */
+export const GET: RequestHandler = async ({ locals }) => {
+	if (!locals.user || locals.user.role !== 'admin') {
+		throw error(403, 'Forbidden');
+	}
+	const res = await backendFetch('/api/admin/import', { method: 'GET' });
+	const data = await res.json().catch(() => ({ tasks: [] }));
+	return json(data);
+};
+
+/**
+ * POST /api/admin/import
+ * Create a new import task.
+ */
+export const POST: RequestHandler = async ({ request, locals }) => {
+	if (!locals.user || locals.user.role !== 'admin') {
+		throw error(403, 'Forbidden');
+	}
+	const body = await request.json();
+	const res = await backendFetch('/api/admin/import', {
+		method: 'POST',
+		headers: { 'Content-Type': 'application/json' },
+		body: JSON.stringify(body)
+	});
+	if (!res.ok) {
+		const err = await res.json().catch(() => ({ error: 'Failed to create import task' }));
+		throw error(res.status, err.error || 'Failed to create import task');
+	}
+	const data = await res.json();
+	return json(data);
+};