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_database.py

@@ -0,0 +1,73 @@
+"""Test database initialization and basic operations."""
+import pytest
+import os
+import tempfile
+from sqlmodel import SQLModel
+
+from app.data.database import init_db, get_session, engine
+from app.data.models import Session, Note, Link
+
+
+class TestDatabase:
+    """Test database operations."""
+    
+    @pytest.mark.asyncio
+    async def test_init_db(self):
+        """Test database initialization creates tables."""
+        await init_db()
+        
+        # Check that tables exist by attempting to create a session
+        async with get_session() as db:
+            session = Session(status="test")
+            db.add(session)
+            await db.commit()
+            await db.refresh(session)
+            assert session.id is not None
+    
+    @pytest.mark.asyncio
+    async def test_session_context_manager(self):
+        """Test database session context manager."""
+        await init_db()
+        
+        # Test successful transaction
+        async with get_session() as db:
+            session = Session(status="test")
+            db.add(session)
+            # Context manager should auto-commit
+        
+        # Verify data was saved
+        async with get_session() as db:
+            from sqlmodel import select
+            statement = select(Session).where(Session.status == "test")
+            result = await db.exec(statement)
+            found_session = result.first()
+            assert found_session is not None
+            assert found_session.status == "test"
+    
+    @pytest.mark.asyncio
+    async def test_session_rollback_on_error(self):
+        """Test that session rolls back on errors."""
+        await init_db()
+        
+        initial_count = 0
+        async with get_session() as db:
+            from sqlmodel import select, func
+            result = await db.exec(select(func.count(Session.id)))
+            initial_count = result.one()
+        
+        # Simulate an error during transaction
+        try:
+            async with get_session() as db:
+                session = Session(status="test_rollback")
+                db.add(session)
+                # Force an error
+                raise ValueError("Simulated error")
+        except ValueError:
+            pass
+        
+        # Verify data was not saved due to rollback
+        async with get_session() as db:
+            from sqlmodel import select, func
+            result = await db.exec(select(func.count(Session.id)))
+            final_count = result.one()
+            assert final_count == initial_count  # No new records