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/package.json

@@ -14,6 +14,9 @@
     "cors": "^2.8.6",
     "dotenv": "^17.3.1",
     "express": "^5.2.1",
+    "multer": "^2.1.1",
+    "openai": "^6.27.0",
+    "pdf-parse": "^2.4.5",
     "pg": "^8.19.0",
     "uuid": "^13.0.0"
   },
@@ -21,6 +24,7 @@
     "@types/cookie-parser": "^1.4.10",
     "@types/cors": "^2.8.19",
     "@types/express": "^5.0.6",
+    "@types/multer": "^2.1.0",
     "@types/node": "^25.3.3",
     "@types/pg": "^8.18.0",
     "@types/uuid": "^10.0.0",