feat: implement data layer with comprehensive test infrastructure

- Define SQLModel schemas for Session, Note, and Link entities
  - Add API request/response models for RPC endpoints
  - Create LLM structured output models for Zettel extraction
  - Set up async database initialization with SQLModel and aiosqlite
  - Implement repository pattern for CRUD operations
  - Add complete test suite with pytest configuration
  - Create validation test runner for development workflow
  - Add .gitignore for Python/FastAPI project security

requirements.txt

@@ -0,0 +1,13 @@
+fastapi
+uvicorn[standard]
+pydantic-settings
+sqlmodel
+aiosqlite
+langchain
+langchain-google-genai
+langchain-community
+chromadb
+tiktoken
+pytest
+pytest-asyncio
+httpx