feat: add intelligent backend routing and model service

- Add modelService for centralized model management
  - Support dynamic model list from config file (data/supported_models.json)
  - Include 2025 latest models: GPT-4.1, o3, o4-mini, Gemini 2.5, etc.
  - File watcher for hot-reload configuration changes

- Improve model detection logic in api.js
  - Priority: modelService lookup → prefix matching fallback
  - Smart backend routing based on model provider

- Add intelligent routing endpoints
  - /v1/chat/completions: unified OpenAI-compatible endpoint
  - /v1/completions: legacy format support
  - Auto-route to Claude/OpenAI/Gemini based on requested model

- Add Xcode system prompt support in openaiToClaude
  - Detect and preserve Xcode-specific system messages

- Export handler functions for reuse
  - openaiClaudeRoutes: export handleChatCompletion
  - openaiRoutes: export handleResponses

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/routes/openaiRoutes.js

@@ -919,3 +919,4 @@ router.get('/key-info', authenticateApiKey, async (req, res) => {
 })
 
 module.exports = router
+module.exports.handleResponses = handleResponses