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

run_tests.py

@@ -0,0 +1,132 @@
+#!/usr/bin/env python
+"""
+Test runner script for SkyTalk API.
+
+This script can be run without pytest to validate basic functionality.
+"""
+import asyncio
+import sys
+from pathlib import Path
+
+# Add the app directory to Python path
+app_dir = Path(__file__).parent
+sys.path.insert(0, str(app_dir))
+
+
+async def test_basic_setup():
+    """Test basic setup and imports."""
+    print("🧪 Testing basic setup...")
+    
+    try:
+        from app.core.config import settings
+        print(f"✅ Config loaded: API key {'set' if settings.GOOGLE_API_KEY and settings.GOOGLE_API_KEY != 'your_google_api_key_here' else 'NOT set'}")
+        
+        from app.data.models import Session, Note, Link, StartSessionRequest
+        print("✅ All models imported successfully")
+        
+        from app.data.database import init_db, get_session
+        print("✅ Database modules imported successfully")
+        
+        from app.data.repositories import SessionRepository, NoteRepository, LinkRepository
+        print("✅ Repository modules imported successfully")
+        
+    except Exception as e:
+        print(f"❌ Import error: {e}")
+        return False
+    
+    return True
+
+
+async def test_model_validation():
+    """Test model creation and validation."""
+    print("\n🧪 Testing model validation...")
+    
+    try:
+        from app.data.models import StartSessionRequest, Session, Note
+        import uuid
+        
+        # Test API models
+        req = StartSessionRequest(topic="AI Ethics")
+        print(f"✅ StartSessionRequest: {req.topic}")
+        
+        # Test database models
+        session = Session()
+        print(f"✅ Session created with ID: {session.id}")
+        
+        note = Note(
+            title="Test Note",
+            content="Test content",
+            tags=["test"],
+            session_id=session.id
+        )
+        print(f"✅ Note created with ID: {note.id}")
+        
+    except Exception as e:
+        print(f"❌ Model validation error: {e}")
+        return False
+    
+    return True
+
+
+async def test_database_operations():
+    """Test database initialization and basic operations."""
+    print("\n🧪 Testing database operations...")
+    
+    try:
+        from app.data.database import init_db, get_session
+        from app.data.repositories import SessionRepository
+        from app.data.models import Session
+        
+        # Initialize database
+        await init_db()
+        print("✅ Database tables created")
+        
+        # Test repository operations
+        async with get_session() as db:
+            repo = SessionRepository(db)
+            session = await repo.create(status="test")
+            print(f"✅ Session created via repository: {session.id}")
+            
+            retrieved = await repo.get(session.id)
+            print(f"✅ Session retrieved: {retrieved.status}")
+        
+    except Exception as e:
+        print(f"❌ Database operation error: {e}")
+        return False
+    
+    return True
+
+
+async def main():
+    """Run all tests."""
+    print("🚀 Running SkyTalk API validation tests...\n")
+    
+    tests = [
+        test_basic_setup,
+        test_model_validation,
+        test_database_operations
+    ]
+    
+    passed = 0
+    total = len(tests)
+    
+    for test in tests:
+        if await test():
+            passed += 1
+        else:
+            break  # Stop on first failure
+    
+    print(f"\n📊 Test Results: {passed}/{total} passed")
+    
+    if passed == total:
+        print("🎉 All basic validation tests passed!")
+        print("\nNext steps:")
+        print("1. Run full test suite: pytest")
+        print("2. Continue with Phase 3 implementation")
+    else:
+        print("❌ Some tests failed. Please fix issues before continuing.")
+        sys.exit(1)
+
+
+if __name__ == "__main__":
+    asyncio.run(main())