fix: forward multipart/form-data correctly in import API proxy

The SvelteKit proxy was calling request.json() unconditionally,
consuming the body before forwarding. File uploads (multipart/form-data)
now use request.formData() and pass the FormData directly to backendFetch
so the fetch API sets the correct Content-Type boundary automatically.

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

@@ -17,22 +17,41 @@ export const GET: RequestHandler = async ({ locals }) => {
 
 /**
  * POST /api/admin/import
- * Create a new import task.
+ * Create a new import task. Supports both multipart/form-data (file upload)
+ * and application/json (object key reference).
  */
 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', {
+	const ct = request.headers.get('content-type') ?? '';
-		method: 'POST',
+
-		headers: { 'Content-Type': 'application/json' },
+	let res: Response;
-		body: JSON.stringify(body)
+
-	});
+	if (ct.includes('multipart/form-data')) {
+		// Forward the raw FormData body; let the browser-set Content-Type
+		// (which includes the boundary) pass through unchanged.
+		const formData = await request.formData();
+		res = await backendFetch('/api/admin/import', {
+			method: 'POST',
+			// Do NOT set Content-Type manually — the fetch API sets it
+			// automatically with the correct boundary when given a FormData body.
+			body: formData
+		});
+	} else {
+		const body = await request.json();
+		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);
-};
+};