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

internal/server/logging.go

@@ -12,6 +12,7 @@ type RequestLog struct {
 	ClientID         string    `json:"client_id"`
 	Provider         string    `json:"provider"`
 	Model            string    `json:"model"`
+	ModelGroup       string    `json:"model_group,omitempty"`
 	PromptTokens     uint32    `json:"prompt_tokens"`
 	CompletionTokens uint32    `json:"completion_tokens"`
 	ReasoningTokens  uint32    `json:"reasoning_tokens"`