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

tests/test_config.py

@@ -0,0 +1,24 @@
+"""Test configuration and environment setup."""
+import pytest
+from app.core.config import settings
+
+
+def test_config_loads():
+    """Test that configuration loads successfully."""
+    assert settings.GOOGLE_API_KEY is not None
+    assert settings.DATABASE_URL.startswith("sqlite+aiosqlite://")
+    assert settings.CHROMA_PERSIST_DIR == "./chroma_db"
+
+
+def test_model_names():
+    """Test that model names are configured correctly."""
+    assert "gemini" in settings.LLM_FLASH_MODEL.lower()
+    assert "gemini" in settings.LLM_PRO_MODEL.lower()
+    assert "embedding" in settings.EMBEDDING_MODEL.lower()
+
+
+def test_api_settings():
+    """Test API configuration."""
+    assert isinstance(settings.API_PORT, int)
+    assert settings.API_PORT > 0
+    assert settings.API_HOST is not None