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

docker-compose.yml

@@ -1,6 +1,7 @@
 # Docker Compose - LogWhisperer AI Development Environment
 # Usage: docker compose up -d
-# Access: http://localhost:5173
+# Access Frontend: http://localhost:5173
+# Access Fake Backend API: http://localhost:3000
 
 services:
   frontend:
@@ -18,8 +19,11 @@ services:
     environment:
       - NODE_ENV=development
       - CHOKIDAR_USEPOLLING=true
+      - VITE_API_URL=http://fake-backend:3000
     # Ensure container restarts on failure
     restart: unless-stopped
+    depends_on:
+      - fake-backend
     # Health check to verify the service is running
     healthcheck:
       test: ["CMD", "wget", "-q", "--spider", "http://localhost:5173"]
@@ -28,6 +32,25 @@ services:
       retries: 3
       start_period: 40s
 
+  fake-backend:
+    build:
+      context: ./tools/fake-backend
+      dockerfile: Dockerfile
+    container_name: logwhisperer-fake-backend
+    ports:
+      - "3000:3000"
+    environment:
+      - PORT=3000
+      - DELAY_MS=1500
+      - NODE_ENV=production
+    restart: unless-stopped
+    healthcheck:
+      test: ["CMD", "node", "-e", "require('http').get('http://localhost:3000/health', (r) => r.statusCode === 200 ? process.exit(0) : process.exit(1))"]
+      interval: 30s
+      timeout: 3s
+      retries: 3
+      start_period: 5s
+
 volumes:
   node_modules:
     driver: local