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

backend/src/config.ts

@@ -18,4 +18,10 @@ export const config = {
   appUserPassword: process.env.APP_USER_PASSWORD || 'changeme',
 
   sessionTimeoutMs: parseInt(process.env.SESSION_TIMEOUT_MS || '10800000', 10),
+
+  /** API-ключ для LLM (OpenAI или совместимый). Обязателен для конвертации PDF. */
+  llmApiKey: process.env.LLM_API_KEY || '',
+
+  /** Базовый URL API LLM. По умолчанию https://api.openai.com. Для Ollama: http://localhost:11434/v1 */
+  llmApiBaseUrl: process.env.LLM_API_BASE_URL || undefined,
 };