feat(api): implement notebook management CRUD endpoints

Implement Sprint 1: Notebook Management CRUD

- Add NotebookService with full CRUD operations
- Add POST /api/v1/notebooks (create notebook)
- Add GET /api/v1/notebooks (list with pagination)
- Add GET /api/v1/notebooks/{id} (get by ID)
- Add PATCH /api/v1/notebooks/{id} (partial update)
- Add DELETE /api/v1/notebooks/{id} (delete)
- Add Pydantic models for requests/responses
- Add custom exceptions (ValidationError, NotFoundError, NotebookLMError)
- Add comprehensive unit tests (31 tests, 97% coverage)
- Add API integration tests (26 tests)
- Fix router prefix duplication
- Fix JSON serialization in error responses

BREAKING CHANGE: None

tests/unit/test_core/test_config.py

@@ -0,0 +1,102 @@
+"""Tests for core configuration."""
+
+import os
+from unittest.mock import patch
+
+import pytest
+
+from notebooklm_agent.core.config import Settings, get_settings
+from notebooklm_agent.core.exceptions import NotebookLMAgentError, ValidationError
+
+
+@pytest.mark.unit
+class TestSettings:
+    """Test suite for Settings configuration."""
+
+    def test_default_values(self):
+        """Should create settings with default values."""
+        # Arrange & Act
+        settings = Settings()
+
+        # Assert
+        assert settings.port == 8000
+        assert settings.host == "0.0.0.0"
+        assert settings.log_level == "INFO"
+        assert settings.debug is False
+        assert settings.testing is False
+
+    def test_custom_values_from_env(self, monkeypatch):
+        """Should load custom values from environment variables."""
+        # Arrange
+        monkeypatch.setenv("NOTEBOOKLM_AGENT_PORT", "9000")
+        monkeypatch.setenv("NOTEBOOKLM_AGENT_HOST", "127.0.0.1")
+        monkeypatch.setenv("LOG_LEVEL", "DEBUG")
+        monkeypatch.setenv("DEBUG", "true")
+
+        # Act
+        settings = Settings()
+
+        # Assert
+        assert settings.port == 9000
+        assert settings.host == "127.0.0.1"
+        assert settings.log_level == "DEBUG"
+        assert settings.debug is True
+
+    def test_cors_origins_parsing_from_string(self):
+        """Should parse CORS origins from comma-separated string."""
+        # Arrange & Act
+        settings = Settings(cors_origins="http://localhost:3000, http://localhost:8080")
+
+        # Assert
+        assert settings.cors_origins == ["http://localhost:3000", "http://localhost:8080"]
+
+    def test_cors_origins_from_list(self):
+        """Should accept CORS origins as list."""
+        # Arrange & Act
+        origins = ["http://localhost:3000", "http://localhost:8080"]
+        settings = Settings(cors_origins=origins)
+
+        # Assert
+        assert settings.cors_origins == origins
+
+    def test_log_level_validation_valid(self):
+        """Should accept valid log levels."""
+        # Arrange & Act & Assert
+        for level in ["DEBUG", "INFO", "WARNING", "ERROR", "CRITICAL"]:
+            settings = Settings(log_level=level)
+            assert settings.log_level == level
+
+    def test_log_level_validation_invalid(self):
+        """Should reject invalid log levels."""
+        # Arrange & Act & Assert
+        with pytest.raises(ValueError, match="log_level must be one of"):
+            Settings(log_level="INVALID")
+
+    def test_is_production_property(self):
+        """Should correctly identify production mode."""
+        # Arrange & Act & Assert
+        assert Settings(debug=False, testing=False).is_production is True
+        assert Settings(debug=True, testing=False).is_production is False
+        assert Settings(debug=False, testing=True).is_production is False
+
+
+@pytest.mark.unit
+class TestGetSettings:
+    """Test suite for get_settings function."""
+
+    def test_returns_settings_instance(self):
+        """Should return a Settings instance."""
+        # Arrange & Act
+        settings = get_settings()
+
+        # Assert
+        assert isinstance(settings, Settings)
+
+    def test_caching(self):
+        """Should cache settings instance."""
+        # Arrange & Act
+        settings1 = get_settings()
+        settings2 = get_settings()
+
+        # Assert - same instance due to lru_cache
+        assert settings1 is settings2