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

.claude/settings.local.json

@@ -0,0 +1,12 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(mkdir:*)",
+      "Bash(python run_tests.py:*)",
+      "Bash(source:*)",
+      "Bash(pytest:*)"
+    ],
+    "deny": [],
+    "ask": []
+  }
+}