Add configurable CORS support for frontend integration

[ademboukabes]

Summary

This PR adds configurable CORS support so browser-based frontends can call the mAIcro API from a different origin.

Changes

• add CORS_ORIGINS configuration support
• expose parsed CORS origins through config.py
• integrate FastAPI CORSMiddleware in main.py
• allow requests only from configured frontend origins
• document the new setting in .env.example and README.md
• add API tests covering:
  • preflight success for an allowed origin
  • rejection for an unconfigured origin

Why

Previously, browser-based frontends such as React, Next.js, or Vite apps running on a different port or domain could not call the backend because the API did not return the required CORS headers.

This change enables frontend integration while keeping cross-origin access explicitly configurable.