Add example/ folder with sample Flask project for demo

[Nehal-cell]

What

Add an example/ folder to the repo with a small Flask project that users can run CodeMorph on to see it work immediately.

Why

New users need a working demo they can try in under 5 minutes. Without it, the barrier to first use is too high.

Acceptance criteria

• example/flask_app/ contains a small Flask app with at least 3 routes
• example/flask_app/tests/ contains passing pytest tests
• README updated with: codemorph migrate --from flask --to fastapi ./example/flask_app

[Nehal-cell]

Added example/flask_app/ with a 3-route Flask CRUD API and 5 passing Pytest tests covering GET, POST, and 404 handling.
Users can now run CodeMorph on a real working project in under 2 minutes:
codemorph migrate --from flask --to fastapi ./example/flask_app
Kept the app simple enough to understand at a glance but complex enough to demonstrate meaningful migration — includes route handlers, error responses, and request body parsing.
Commit: https://github.com/Nehal-cell/CodeMorph/tree/main/example/flask_app