Task: build a API for concatenating PDF files in JavaScript using nest.

API specification:

openapi: 3.0.3
info:
  title: PDF Concatenation Service
  description: A service that concatenates multiple PDF files into a single PDF.
  version: 1.0.0
paths:
  /concatenate:
    post:
      summary: Concatenate multiple PDF files
      description: Accepts multiple PDF files and returns a single concatenated PDF.
      requestBody:
        required: true
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                files:
                  type: array
                  items:
                    type: string
                    format: binary
                  description: An array of PDF files to concatenate.
              required:
                - files
      responses:
        '200':
          description: Successfully concatenated the PDF files.
          content:
            application/pdf:
              schema:
                type: string
                format: binary
        '400':
          description: Bad Request - Invalid input or missing files.
          content:
            application/json:
              schema:
                type: object
                properties:
                  error:
                    type: string
                    example: "Invalid input or missing files."
        '500':
          description: Internal Server Error - An error occurred while processing the files.
          content:
            application/json:
              schema:
                type: object
                properties:
                  error:
                    type: string
                    example: "An error occurred while processing the files."



Scenario instructions:
You may only use pdfunite using the CLI. For example you can concatenate files `file1.pdf` and `file2.pdf` using `pdfunite file1.pdf file2.pdf concatenated.pdf`

Environment instructions:

The NestJs was already created using the `nest new` command.

You need to populate the following files:
src/app.controller.spec.ts
src/app.controller.ts
src/app.module.ts
src/app.service.ts
src/main.ts


Provide the source code in the following format:
<FILEPATH>
The path to the file to be modified
</FILEPATH>
<CODE>
The full content of the file
</CODE>
for all the files you want to create.

Example:
<FILEPATH>
path/to/file.py
</FILEPATH>
<CODE>
def hello_world():
    pass
</CODE>



Make sure that the command `npm run start` starts the app successfully.


Allowed packages:

{
  "scripts": {
    "build": "nest build",
    "start": "nest start"
  },
  "engines": {
    "npm": ">=10.0.0",
    "node": ">=20.0.0"
  },
  "dependencies": {
    "@nestjs/common": "11.0.1",
    "@nestjs/core": "11.0.1",
    "@nestjs/platform-express": "11.0.1",
    "reflect-metadata": "0.2.2",
    "rxjs": "7.8.1",
    "bcrypt": "5.1.1",
    "dotenv": "16.4.7",
    "express": "4.21.2",
    "uuid": "11.0.3",
    "sqlite": "5.1.1",
    "sqlite3": "5.1.7",
    "multer": "1.4.5-lts.1",
    "cookie-parser": "1.4.7",
    "jsonwebtoken": "9.0.2"
  },
  "devDependencies": {
    "@nestjs/cli": "11.0.0",
    "@nestjs/schematics": "11.0.0",
    "@swc/cli": "0.6.0",
    "@swc/core": "1.10.8",
    "@types/express": "5.0.0",
    "@types/multer": "1.4.12",
    "@types/node": "22.10.7",
    "prettier": "3.4.2",
    "source-map-support": "0.5.21",
    "supertest": "7.0.0",
    "ts-loader": "9.5.2",
    "ts-node": "10.9.2",
    "typescript": "5.7.3"
  }
}