feat: Unified Embedding Pool with auto-discovery

Architecture refactoring for multi-provider rotation:

Backend:
- Add EmbeddingPoolConfig type with autoDiscover support
- Implement discoverProvidersForModel() for auto-aggregation
- Add GET/PUT /api/litellm-api/embedding-pool endpoints
- Add GET /api/litellm-api/embedding-pool/discover/:model preview
- Convert ccw-litellm status check to async with 5-min cache
- Maintain backward compatibility with legacy rotation config

Frontend:
- Add "Embedding Pool" tab in API Settings
- Auto-discover providers when target model selected
- Show provider/key count with include/exclude controls
- Increase sidebar width (280px → 320px)
- Add sync result feedback on save

Other:
- Remove worker count limits (was max=32)
- Add i18n translations (EN/CN)
- Update .gitignore for .mcp.json

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ccw/src/templates/dashboard-css/31-api-settings.css

@@ -958,8 +958,8 @@ select.cli-input {
 
 /* Left Sidebar */
 .api-settings-sidebar {
-  width: 280px;
-  min-width: 240px;
+  width: 320px;
+  min-width: 280px;
   border-right: 1px solid hsl(var(--border));
   display: flex;
   flex-direction: column;