feat: Add multi-LLM provider support with DeepSeek integration

Major Features:
- ✨ Multi-LLM provider support (DeepSeek, Ollama, OpenAI, Custom)
- 🤖 Admin panel LLM configuration management UI
- 🔄 Dynamic provider switching without restart
- 🧪 Built-in API connection testing
- 🔒 Secure API key management

Backend Changes:
- Add routes/llmConfig.js: Complete LLM config CRUD API
- Update routes/analyze.js: Use database LLM configuration
- Update server.js: Add LLM config routes
- Add scripts/add-deepseek-config.js: DeepSeek setup script

Frontend Changes:
- Update src/pages/AdminPage.jsx: Add LLM Config tab + modal
- Update src/services/api.js: Add LLM config API methods
- Provider presets for DeepSeek, Ollama, OpenAI
- Test connection feature in config modal

Configuration:
- Update .env.example: Add DeepSeek API configuration
- Update package.json: Add llm:add-deepseek script

Documentation:
- Add docs/LLM_CONFIGURATION_GUIDE.md: Complete guide
- Add DEEPSEEK_INTEGRATION.md: Integration summary
- Quick setup instructions for DeepSeek

API Endpoints:
- GET /api/llm-config: List all configurations
- GET /api/llm-config/active: Get active configuration
- POST /api/llm-config: Create configuration
- PUT /api/llm-config/🆔 Update configuration
- PUT /api/llm-config/:id/activate: Activate configuration
- DELETE /api/llm-config/🆔 Delete configuration
- POST /api/llm-config/test: Test API connection

Database:
- Uses existing llm_configs table
- Only one config active at a time
- Fallback to Ollama if no database config

Security:
- Admin-only access to LLM configuration
- API keys never returned in GET requests
- Audit logging for all config changes
- Cannot delete active configuration

DeepSeek Model:
- Model: deepseek-chat
- High-quality 5 Why analysis
- Excellent Chinese language support
- Cost-effective pricing

🤖 Generated with Claude Code
Co-Authored-By: Claude <noreply@anthropic.com>

.env.example

@@ -10,13 +10,18 @@ SERVER_HOST=localhost
 SERVER_PORT=3001
 CLIENT_PORT=5173
 
-# Ollama API Configuration
+# Ollama API Configuration (Fallback if no database config)
 OLLAMA_API_URL=https://ollama_pjapi.theaken.com
 OLLAMA_MODEL=qwen2.5:3b
 
-# LLM API Keys (Optional - for admin configuration)
-GEMINI_API_KEY=
+# DeepSeek API Configuration (Recommended)
+# Get your API key from: https://platform.deepseek.com/
+DEEPSEEK_API_URL=https://api.deepseek.com
 DEEPSEEK_API_KEY=
+DEEPSEEK_MODEL=deepseek-chat
+
+# Other LLM API Keys (Optional - for admin configuration)
+GEMINI_API_KEY=
 OPENAI_API_KEY=
 
 # Session Secret (Generate a random string)