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>

src/services/api.js

@@ -170,6 +170,38 @@ class ApiClient {
     return this.get('/api/admin/statistics');
   }
 
+  // ============================================
+  // LLM Configuration APIs
+  // ============================================
+
+  async getLLMConfigs() {
+    return this.get('/api/llm-config');
+  }
+
+  async getActiveLLMConfig() {
+    return this.get('/api/llm-config/active');
+  }
+
+  async createLLMConfig(configData) {
+    return this.post('/api/llm-config', configData);
+  }
+
+  async updateLLMConfig(id, configData) {
+    return this.put(`/api/llm-config/${id}`, configData);
+  }
+
+  async activateLLMConfig(id) {
+    return this.put(`/api/llm-config/${id}/activate`, {});
+  }
+
+  async deleteLLMConfig(id) {
+    return this.delete(`/api/llm-config/${id}`);
+  }
+
+  async testLLMConfig(configData) {
+    return this.post('/api/llm-config/test', configData);
+  }
+
   // ============================================
   // Health Check
   // ============================================