docs: add .env.example for Claude AI setup

Includes:
- OPENAI_API_KEY for zunef gateway (OpenAI-compatible)
- DEFAULT_MODEL=claude-sonnet-4-6
- Optional ANTHROPIC_API_KEY for direct Anthropic API
- Other services (Redis, StarRocks, Langfuse)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/.env.example

+# Claude API via Zunef Gateway (OpenAI-compatible endpoint)
+OPENAI_API_KEY=your_zunef_api_key_here
+DEFAULT_MODEL=claude-sonnet-4-6
+
+# Alternative: Direct Anthropic API (if not using zunef)
+# ANTHROPIC_API_KEY=your_anthropic_key_here
+
+# Other APIs (optional)
+DS2API_BASE_URL=http://your-ds2-api:port
+DS2API_API_KEY=your_ds2_key
+GORQ_API_KEY=your_gorq_key
+GOOGLE_API_KEY=your_google_key
+
+# Server
+PORT=5000
+
+# Redis
+REDIS_HOST=127.0.0.1
+REDIS_PORT=6379
+
+# StarRocks DB
+STARROCKS_HOST=your_starrocks_host
+STARROCKS_PORT=9030
+
+# Langfuse Tracing (optional)
+LANGFUSE_TRACING=false
+LANGFUSE_PUBLIC_KEY=
+LANGFUSE_SECRET_KEY=
+
+# Disable Auth (for development)
+DISABLE_AUTH=true