feat(backend): FastAPI + libSQL + auth register/login/me + tests (TDD)

- FastAPI app with lifespan, CORS, health endpoint
- SQLAlchemy 2.0 async with aiosqlite, Base/UUIDMixin/TenantMixin/TimestampMixin
- Tenant and User models with multi-tenant isolation
- Auth: register (creates tenant+user), login, /me endpoint
- JWT HS256 tokens, bcrypt password hashing
- Alembic async setup with initial migration
- 6 passing tests (register, login, wrong password, me, no token, health)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/requirements.txt

@@ -0,0 +1,14 @@
+fastapi>=0.115
+uvicorn[standard]>=0.30
+sqlalchemy>=2.0
+aiosqlite>=0.20
+alembic>=1.13
+python-jose[cryptography]>=3.3
+passlib[bcrypt]>=1.7
+pydantic-settings>=2.0
+pydantic[email]>=2.0
+pytest>=8.0
+pytest-asyncio>=0.23
+httpx>=0.27
+weasyprint>=62
+jinja2>=3.1