Add REST API server mode

[ahmed-sekka]

Description

Add an optional REST API server to enable integration with web applications and other tools.

Proposed command

ragctl serve --port 8000 --host 0.0.0.0

API Endpoints

POST /api/v1/chunk          - Process single document
POST /api/v1/batch          - Process multiple documents (async)
GET  /api/v1/jobs/{id}      - Get job status
GET  /api/v1/health         - Health check
GET  /api/v1/info           - System info
GET  /docs                  - OpenAPI documentation

Tasks

• Add fastapi and uvicorn to optional dependencies
• Create src/core/api/ module
• Implement endpoints with proper error handling
• Add OpenAPI/Swagger documentation
• Add ragctl serve command
• Add rate limiting option
• Write API tests
• Add Docker support

Example request

curl -X POST http://localhost:8000/api/v1/chunk \
  -F "file=@document.pdf" \
  -F "strategy=semantic" \
  -F "max_tokens=400"