Initial project structure

Scaffold all modules, route stubs, data models, and config.
No logic implemented yet — all core methods raise NotImplementedError.
Establishes the full directory layout matching the architecture in CLAUDE.md.

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

requirements.txt

@@ -0,0 +1,10 @@
+fastapi>=0.111.0
+uvicorn[standard]>=0.29.0
+pydantic>=2.7.0
+pydantic-settings>=2.2.0
+httpx>=0.27.0
+aiosqlite>=0.20.0
+python-dotenv>=1.0.0
+tiktoken>=0.7.0
+pytest>=8.0.0
+pytest-asyncio>=0.23.0