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

docs/README.md

@@ -0,0 +1,71 @@
+# Documentation
+
+Benvenuto nella documentazione di NotebookLM Agent API.
+
+## Indice
+
+- [API Reference](./api/) - Documentazione completa delle API (TODO)
+- [Examples](./examples/) - Esempi di utilizzo (TODO)
+
+## Panoramica
+
+NotebookLM Agent API fornisce:
+
+1. **REST API** per gestire notebook, fonti, chat e generazione contenuti
+2. **Webhook System** per notifiche event-driven
+3. **AI Skill** per integrazione con agenti AI
+
+## Endpoint Principali
+
+### Notebook Management
+- `POST /api/v1/notebooks` - Creare notebook
+- `GET /api/v1/notebooks` - Listare notebook
+- `GET /api/v1/notebooks/{id}` - Ottenere notebook
+- `DELETE /api/v1/notebooks/{id}` - Eliminare notebook
+
+### Source Management
+- `POST /api/v1/notebooks/{id}/sources` - Aggiungere fonte
+- `GET /api/v1/notebooks/{id}/sources` - Listare fonti
+- `POST /api/v1/notebooks/{id}/sources/research` - Ricerca web
+
+### Content Generation
+- `POST /api/v1/notebooks/{id}/generate/audio` - Generare podcast
+- `POST /api/v1/notebooks/{id}/generate/video` - Generare video
+- `POST /api/v1/notebooks/{id}/generate/quiz` - Generare quiz
+- `POST /api/v1/notebooks/{id}/generate/flashcards` - Generare flashcard
+
+### Webhooks
+- `POST /api/v1/webhooks` - Registrare webhook
+- `GET /api/v1/webhooks` - Listare webhook
+- `POST /api/v1/webhooks/{id}/test` - Testare webhook
+
+## Autenticazione
+
+Tutte le richieste API richiedono header `X-API-Key`:
+
+```bash
+curl http://localhost:8000/api/v1/notebooks \
+  -H "X-API-Key: your-api-key"
+```
+
+## Webhook Security
+
+I webhook includono firma HMAC-SHA256 nell'header `X-Webhook-Signature`:
+
+```python
+import hmac
+import hashlib
+
+signature = hmac.new(
+    secret.encode(),
+    payload.encode(),
+    hashlib.sha256
+).hexdigest()
+```
+
+## Risorse
+
+- [README](../README.md) - Panoramica progetto
+- [PRD](../prd.md) - Requisiti prodotto
+- [SKILL.md](../SKILL.md) - Skill per agenti AI
+- [CONTRIBUTING](../CONTRIBUTING.md) - Come contribuire