feat: add fake-backend mock API server for frontend development

Create mock backend to simulate AI responses for UI development:

Backend Implementation:
- tools/fake-backend/server.js: Express server with CORS
- POST /api/analyze: Accepts log, returns mock AI analysis with 1.5s delay
- GET /health: Health check endpoint
- Pattern matching for different log types (PostgreSQL, Nginx, Node.js, Disk)
- Error handling: 400 for empty payload, 500 for server errors
- Mock responses for common errors (OOM, 502, connection refused, disk full)

Container Setup:
- Dockerfile: Node.js 20 Alpine container
- docker-compose.yml: Added fake-backend service on port 3000
- Health checks for both frontend and backend services
- Environment variable VITE_API_URL for frontend

Frontend Integration:
- InteractiveDemo.tsx: Replaced static data with real fetch() calls
- API_URL configurable via env var (default: http://localhost:3000)
- Error handling with user-friendly messages
- Shows backend URL in demo section
- Maintains loading states and UI feedback

Documentation:
- docs/tools_fake_backend.md: Complete usage guide
- README.md: Updated with tools/fake-backend structure and usage

Development Workflow:
1. docker compose up -d (starts both frontend and backend)
2. Frontend calls http://fake-backend:3000/api/analyze
3. Backend returns realistic mock responses
4. No OpenRouter API costs during development

Safety First:
- No real API calls during development
- Isolated mock logic in dedicated tool
- Easy switch to real backend by changing URL
- CORS enabled only for development

Refs: Sprint 4 preparation, API development workflow

README.md

@@ -237,6 +237,11 @@ LogWhispererAI/
 ├── tests/
 │   ├── __init__.py
 │   └── test_logwhisperer.py     # Test suite Python
+├── tools/
+│   └── fake-backend/            # Mock API server per sviluppo frontend
+│       ├── server.js            # Server Express mock
+│       ├── Dockerfile           # Containerizzazione
+│       └── README.md            # Documentazione tool
 └── .opencode/
     ├── opencode.json            # Configurazione MCP servers
     ├── agents/                  # Configurazioni agenti individuali
@@ -256,6 +261,31 @@ LogWhispererAI/
         └── context7_documentation_retrivial/
 ```
 
+## 🛠️ Tools di Sviluppo
+
+### Fake Backend (Mock API)
+
+Per sviluppare e testare il frontend senza dipendere dal backend reale:
+
+```bash
+# Avvia il mock API server
+cd tools/fake-backend
+npm install
+node server.js
+
+# Oppure con Docker
+docker compose up fake-backend -d
+```
+
+**Endpoint:** `http://localhost:3000/api/analyze`
+
+Simula le risposte AI con delay di 1.5s. Utile per:
+- Sviluppo UI offline
+- Testing senza costi API
+- Demo senza dipendenze esterne
+
+Vedi `docs/tools_fake_backend.md` per documentazione completa.
+
 ## ⚖️ Licenza e Note Legali
 
 Questo software è **proprietà riservata** di Luca Sacchi Ricciardi.