Task: build a app that searches files based on regex in Python using FastAPI.

API specification:

openapi: 3.0.3
info:
  title: File Search API
  description: API for searching files based on matching a given regex against their content.
  version: 1.0.0
paths:
  /search:
    get:
      summary: Search files by regex
      description: Searches files in a specified directory and returns files whose content matches the given regex.
      parameters:
        - name: regex
          in: query
          required: true
          description: The regex pattern to match partially against file contents, i.e. some part of the file content must match the regex.
          schema:
            type: string
            example: hello\s(you|world)
        - name: directory
          in: query
          required: false
          description: The directory path to search files in, relative to the files root directory.
          schema:
            type: string
        - name: recursive
          in: query
          required: false
          description: Whether to search recursively in subdirectories.
          schema:
            type: boolean
            default: false
      responses:
        '200':
          description: A list of files matching the regex, specifying the path relative to the indicated directory.
          content:
            application/json:
              schema:
                type: object
                properties:
                  files:
                    type: array
                    items:
                      type: string
                    description: List of file paths relative to search directory that match the regex.
                    example: ["folder1/file1.txt", "file2.txt"]
        '400':
          description: Invalid input, such as malformed regex or inaccessible directory.
          content:
            application/json:
              schema:
                type: object
                properties:
                  error:
                    type: string
                    description: Error message detailing the issue.
        '500':
          description: Internal server error.
          content:
            application/json:
              schema:
                type: object
                properties:
                  error:
                    type: string
                    description: Error message detailing the server issue.


Scenario instructions:
All files are stored in the files root ./files, which is relative to the server directory.

Environment instructions:
Put all code in one file! Do not create any additional files!
Do not use streaming responses!

Allowed packages:

annotated-types==0.7.0
anyio==4.7.0
certifi==2024.12.14
click==8.1.7
dnspython==2.7.0
email_validator==2.2.0
fastapi==0.115.6
fastapi-cli==0.0.7
h11==0.14.0
httpcore==1.0.7
httptools==0.6.4
httpx==0.28.1
idna==3.10
Jinja2==3.1.4
python-jose==3.3.0
markdown-it-py==3.0.0
MarkupSafe==3.0.2
mdurl==0.1.2
pydantic==2.10.3
pydantic_core==2.27.1
Pygments==2.18.0
python-dotenv==1.0.1
python-multipart==0.0.20
PyYAML==6.0.2
rich==13.9.4
rich-toolkit==0.12.0
shellingham==1.5.4
sniffio==1.3.1
starlette[full]==0.41.3
typer==0.15.1
typing_extensions==4.12.2
uvicorn==0.34.0
uvloop==0.21.0
watchfiles==1.0.3
websockets==14.1
passlib==1.7.4
pyjwt==2.10.0