fix: log resolved model name instead of group name in Recent Activity

When using model groups (e.g. 'deepseek-auto'), the dashboard logged the
group name instead of the concrete resolved model (e.g. 'deepseek-reasoner').

Now:
- logRequest passes the resolved modelID (concrete) + modelGroup (group name)
- RequestLog struct has a new ModelGroup field (omitempty)
- Dashboard displays resolved model (via group) when a group was used

Files changed:
  internal/server/logging.go  - add ModelGroup field
  internal/server/server.go   - pass resolved modelID, capture modelGroup
  static/js/websocket.js      - show group annotation in Recent Activity
  static/js/pages/overview.js - show group annotation in overview table
  static/js/pages/monitoring.js - show group annotation in stream

static/js/pages/monitoring.js

@@ -392,7 +392,7 @@ class MonitoringPage {
             </div>
             <div class="stream-entry-content">
                 <strong>${request.client_id || 'Unknown'}</strong> → 
-                ${request.provider || 'Unknown'} (${request.model || 'Unknown'})
+                ${request.provider || 'Unknown'} (${request.model || 'Unknown'}${request.model_group ? ` via ${request.model_group}` : ''})
                 <div class="stream-entry-details">
                     ${request.total_tokens || request.tokens || 0} tokens • ${request.duration_ms || request.duration || 0}ms
                 </div>