docs: add .env.example for Claude AI setup

- Simple 2-variable config: OPENAI_API_KEY + DEFAULT_MODEL
- Claude API auto-detected via model name "claude*"
- No code changes needed, just drop key and model name
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/.env.example

+# ==========================================
+# Claude AI via zunef gateway - CANIFA CHATBOT
+# Only need 2 variables to use Claude AI!
+# ==========================================
+
+# Claude API (via zunef.com gateway)
+# The API key is used as OPENAI_API_KEY (for compatibility)
+OPENAI_API_KEY=zf_V8pSCuyYqAWhgxiioDScISIx48Ueq7DC
+
+# Default model - just change this to switch models
+# Options: claude-sonnet-4-6, claude-3-5-sonnet, etc.
+DEFAULT_MODEL=claude-sonnet-4-6
+
+# Claude API base URL (auto-configured, no need to change)
+CLAUDE_BASE_URL=https://claude-api.zunef.com/v1/ai
+
+# ==========================================
+# Other API keys (optional - not needed for Claude)
+# ==========================================
+DS2API_BASE_URL=
+DS2API_API_KEY=
+GOOGLE_API_KEY=
+GROQ_API_KEY=
+
+# ==========================================
+# Server Config
+# ==========================================
+PORT=5000
+REDIS_HOST=172.16.2.192
+REDIS_PORT=6379
+
+# ==========================================
+# Database & Services
+# ==========================================
+STARROCKS_HOST=
+STARROCKS_PORT=9030
+LANGFUSE_TRACING=false
+DISABLE_AUTH=true