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_models.py

@@ -0,0 +1,131 @@
+"""Test data models and validation."""
+import uuid
+from datetime import datetime
+
+import pytest
+from pydantic import ValidationError
+
+from app.data.models import (
+    StartSessionRequest,
+    SendMessageRequest,
+    SessionResponse,
+    SessionStatusResponse,
+    RawZettel,
+    SegmentationResult,
+    RawLink,
+    LinkingResult,
+    Note,
+    Session,
+    Link
+)
+
+
+class TestAPIModels:
+    """Test API request/response models."""
+    
+    def test_start_session_request(self):
+        """Test StartSessionRequest validation."""
+        req = StartSessionRequest(topic="AI Ethics")
+        assert req.topic == "AI Ethics"
+        
+        # Test empty topic fails
+        with pytest.raises(ValidationError):
+            StartSessionRequest(topic="")
+    
+    def test_send_message_request(self):
+        """Test SendMessageRequest validation."""
+        session_id = uuid.uuid4()
+        req = SendMessageRequest(session_id=session_id, message="Hello")
+        assert req.session_id == session_id
+        assert req.message == "Hello"
+        
+        # Test invalid UUID fails
+        with pytest.raises(ValidationError):
+            SendMessageRequest(session_id="invalid", message="Hello")
+    
+    def test_session_response(self):
+        """Test SessionResponse model."""
+        session_id = uuid.uuid4()
+        resp = SessionResponse(
+            session_id=session_id,
+            status="active",
+            message="Welcome!"
+        )
+        assert resp.session_id == session_id
+        assert resp.status == "active"
+        assert resp.message == "Welcome!"
+
+
+class TestLLMModels:
+    """Test LLM structured output models."""
+    
+    def test_raw_zettel(self):
+        """Test RawZettel model."""
+        zettel = RawZettel(
+            title="Test Concept",
+            content="This is a test concept about AI.",
+            tags=["ai", "test", "concept"]
+        )
+        assert zettel.title == "Test Concept"
+        assert len(zettel.tags) == 3
+        assert "ai" in zettel.tags
+    
+    def test_segmentation_result(self):
+        """Test SegmentationResult model."""
+        zettels = [
+            RawZettel(title="Concept 1", content="Content 1", tags=["tag1"]),
+            RawZettel(title="Concept 2", content="Content 2", tags=["tag2"])
+        ]
+        result = SegmentationResult(notes=zettels)
+        assert len(result.notes) == 2
+        assert result.notes[0].title == "Concept 1"
+    
+    def test_raw_link(self):
+        """Test RawLink model."""
+        link = RawLink(
+            target_note_id=str(uuid.uuid4()),
+            relationship_context="Related through shared AI concepts"
+        )
+        assert uuid.UUID(link.target_note_id)  # Should not raise
+        assert "AI" in link.relationship_context
+
+
+class TestDatabaseModels:
+    """Test database models."""
+    
+    def test_session_creation(self):
+        """Test Session model creation."""
+        session = Session()
+        assert isinstance(session.id, uuid.UUID)
+        assert session.status == "active"
+        assert session.transcript == []
+        assert isinstance(session.created_at, datetime)
+    
+    def test_note_creation(self):
+        """Test Note model creation."""
+        session_id = uuid.uuid4()
+        note = Note(
+            title="Test Note",
+            content="This is test content",
+            tags=["test", "note"],
+            session_id=session_id
+        )
+        assert isinstance(note.id, uuid.UUID)
+        assert note.title == "Test Note"
+        assert note.content == "This is test content"
+        assert note.tags == ["test", "note"]
+        assert note.session_id == session_id
+        assert isinstance(note.created_at, datetime)
+    
+    def test_link_creation(self):
+        """Test Link model creation."""
+        source_id = uuid.uuid4()
+        target_id = uuid.uuid4()
+        link = Link(
+            context="These concepts are related through...",
+            source_id=source_id,
+            target_id=target_id
+        )
+        assert link.context == "These concepts are related through..."
+        assert link.source_id == source_id
+        assert link.target_id == target_id