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>

scripts/add-deepseek-config.js

@@ -0,0 +1,88 @@
+#!/usr/bin/env node
+/**
+ * Add DeepSeek LLM Configuration
+ * This script adds a DeepSeek configuration to the llm_configs table
+ */
+
+import { pool, query } from '../config.js';
+import dotenv from 'dotenv';
+
+dotenv.config();
+
+async function addDeepSeekConfig() {
+  console.log('━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━');
+  console.log('  Adding DeepSeek LLM Configuration');
+  console.log('━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━\n');
+
+  try {
+    // Check if DeepSeek config already exists
+    const existing = await query(
+      `SELECT id FROM llm_configs WHERE provider_name = 'DeepSeek' LIMIT 1`
+    );
+
+    if (existing.length > 0) {
+      console.log('✅ DeepSeek configuration already exists (ID:', existing[0].id, ')');
+      console.log('   Skipping...\n');
+      return;
+    }
+
+    // Get API key from environment or leave empty
+    const apiKey = process.env.DEEPSEEK_API_KEY || '';
+
+    if (!apiKey) {
+      console.log('⚠️  Warning: DEEPSEEK_API_KEY not found in .env');
+      console.log('   You will need to add the API key in the admin panel\n');
+    }
+
+    // First, deactivate all existing configs
+    await query('UPDATE llm_configs SET is_active = 0');
+
+    // Insert DeepSeek configuration
+    const result = await query(
+      `INSERT INTO llm_configs
+       (provider_name, api_endpoint, api_key, model_name, temperature, max_tokens, timeout_seconds, is_active)
+       VALUES (?, ?, ?, ?, ?, ?, ?, ?)`,
+      [
+        'DeepSeek',
+        process.env.DEEPSEEK_API_URL || 'https://api.deepseek.com',
+        apiKey || null,
+        process.env.DEEPSEEK_MODEL || 'deepseek-chat',
+        0.7,
+        6000,
+        120,
+        1 // Set as active
+      ]
+    );
+
+    console.log('✅ DeepSeek configuration added successfully!');
+    console.log('   Config ID:', result.insertId);
+    console.log('   Provider: DeepSeek');
+    console.log('   Model: deepseek-chat');
+    console.log('   Status: Active\n');
+
+    console.log('📝 Next steps:');
+    console.log('   1. Go to Admin Panel > LLM 配置');
+    console.log('   2. Add your DeepSeek API key if not already set');
+    console.log('   3. Test the connection');
+    console.log('   4. Start using DeepSeek for 5 Why analysis!\n');
+
+  } catch (error) {
+    console.error('❌ Error adding DeepSeek configuration:', error.message);
+    process.exit(1);
+  } finally {
+    await pool.end();
+  }
+}
+
+// Run the script
+addDeepSeekConfig()
+  .then(() => {
+    console.log('━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━');
+    console.log('  Configuration Complete');
+    console.log('━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━\n');
+    process.exit(0);
+  })
+  .catch((error) => {
+    console.error('Fatal error:', error);
+    process.exit(1);
+  });