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

pytest.ini

@@ -0,0 +1,9 @@
+[tool:pytest]
+asyncio_mode = auto
+testpaths = tests
+python_files = test_*.py
+python_classes = Test*
+python_functions = test_*
+addopts = -v --tb=short
+markers = 
+    asyncio: mark test as an async test