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

.env.example

@@ -0,0 +1,26 @@
+# API Key Configuration
+NOTEBOOKLM_AGENT_API_KEY=your-api-key-here
+NOTEBOOKLM_AGENT_WEBHOOK_SECRET=your-webhook-secret-here
+
+# Server Configuration
+NOTEBOOKLM_AGENT_PORT=8000
+NOTEBOOKLM_AGENT_HOST=0.0.0.0
+NOTEBOOKLM_AGENT_RELOAD=true
+
+# NotebookLM Configuration
+NOTEBOOKLM_HOME=~/.notebooklm
+NOTEBOOKLM_PROFILE=default
+
+# Redis Configuration (for Celery/Queue)
+REDIS_URL=redis://localhost:6379/0
+
+# Logging
+LOG_LEVEL=INFO
+LOG_FORMAT=json
+
+# Development
+DEBUG=true
+TESTING=false
+
+# Security
+CORS_ORIGINS=http://localhost:3000,http://localhost:8080