Add fastapi-fastgen-starter to Boilerplate

[YIbaikaishui]

What

Adds fastapi-fastgen-starter to the Boilerplate section (appended at the end, per contributing.md).

Why it's awesome

• Stays organized as it grows — src/ layout with one vertical-slice module per business domain (domain/ → application/ → infrastructure/ → api/), and an auto-maintained registry (src/modules/__init__.py) that routers auto-mount from, so adding a module never means hand-editing main.py
• Async SQLAlchemy 2.0 done right — Mapped/mapped_column, AsyncAttrs, a single get_session dependency
• Alembic from day one — schema lives in migrations, not create_all
• A worked example — ships a filled-in CRUD module with endpoint tests, not just empty skeletons
• Zero lock-in — plain FastAPI + SQLAlchemy; the template companion to fastgen-cli, a uvx-runnable module manager