feat(backend): add fastapi crud app - #75

Description:

• Small, minimal FastAPI example demonstrating a CRUD REST API for "items".
• Uses SQLAlchemy ORM for persistence (SQLite by default) and Pydantic schemas for request/response validation.

Features:

• CRUD endpoints for items:
  • GET /items/ — list (supports skip/limit pagination)
  • GET /items/{id} — retrieve single item (404 if missing)
  • POST /items/ — create (validates input)
  • PUT /items/{id} — update (404 if missing)
  • DELETE /items/{id} — delete (404 if missing)
• Pydantic request/response schemas returning SQLAlchemy models

Tech Stack:

• Python 3.12+
• FastAPI
• Pydantic (validation / schemas)
• SQLAlchemy (ORM)
• Uvicorn (ASGI server)
• "uv" recommended environment manager for local dev