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

tools/fake-backend/Dockerfile

@@ -0,0 +1,31 @@
+# Dockerfile for Fake Backend
+# Development mock API server for LogWhisperer AI
+
+FROM node:20-alpine
+
+# Set working directory
+WORKDIR /app
+
+# Copy package files
+COPY package*.json ./
+
+# Install dependencies
+RUN npm ci --only=production
+
+# Copy server code
+COPY server.js ./
+
+# Expose port 3000
+EXPOSE 3000
+
+# Set environment variables
+ENV PORT=3000
+ENV DELAY_MS=1500
+ENV NODE_ENV=production
+
+# Health check
+HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
+  CMD node -e "require('http').get('http://localhost:3000/health', (r) => r.statusCode === 200 ? process.exit(0) : process.exit(1))"
+
+# Start server
+CMD ["node", "server.js"]