feat: adds PDF import with conversion to JSON 1.0

- Accept only PDF and JSON files in import modal and API
- Convert PDF statements to JSON 1.0 via LLM (OpenAI-compatible)
- Use multipart/form-data for file upload (multer, 15 MB limit)
- Add LLM_API_KEY and LLM_API_BASE_URL for configurable LLM endpoint
- Update ImportModal to validate type and send FormData
- Add postFormData to API client for file upload

frontend/src/api/client.ts

@@ -53,6 +53,40 @@ async function request<T>(url: string, options: RequestInit = {}): Promise<T> {
   return res.json();
 }
 
+async function requestFormData<T>(url: string, formData: FormData): Promise<T> {
+  const res = await fetch(url, {
+    method: 'POST',
+    body: formData,
+    credentials: 'include',
+    // Do not set Content-Type — browser sets multipart/form-data with boundary
+  });
+
+  if (res.status === 401) {
+    let body: ApiError;
+    try {
+      body = await res.json();
+    } catch {
+      body = { error: 'UNAUTHORIZED', message: 'Сессия истекла' };
+    }
+    if (!url.includes('/api/auth/login')) {
+      onUnauthorized?.();
+    }
+    throw new ApiException(401, body);
+  }
+
+  if (!res.ok) {
+    let body: ApiError;
+    try {
+      body = await res.json();
+    } catch {
+      body = { error: 'UNKNOWN', message: res.statusText };
+    }
+    throw new ApiException(res.status, body);
+  }
+
+  return res.json();
+}
+
 export const api = {
   get: <T>(url: string) => request<T>(url),
 
@@ -62,6 +96,9 @@ export const api = {
       body: body != null ? JSON.stringify(body) : undefined,
     }),
 
+  postFormData: <T>(url: string, formData: FormData) =>
+    requestFormData<T>(url, formData),
+
   put: <T>(url: string, body: unknown) =>
     request<T>(url, { method: 'PUT', body: JSON.stringify(body) }),