feat: audit group AI budget mutations (#25374)

Relates to
https://linear.app/codercom/issue/AIGOV-284/add-group-budgets-table-and-crud-api

Adds audit-log support for `group_ai_budget` mutations. Without it, an
admin could silently lower a spend limit from `$500` to `$50` or delete
a budget entirely, with no record of who performed the action.

Both write (`create-or-update`) and delete actions now produce audit log
entries, including before/after diffs for `spend_limit_micros`.

Depends on #25203.

## Old Version
<img width="1340" height="456" alt="image"
src="https://github.com/user-attachments/assets/e9ff52fb-a905-4aef-a4ee-7cdc58e68b75"
/>

## New Version (see
https://github.com/coder/coder/pull/25374/changes/9d22833de87cc106c24142c1d471a3f71872bf67)
<img width="1347" height="496" alt="image"
src="https://github.com/user-attachments/assets/1b9bbfa1-f86d-48e3-a0b1-266eb76f851f"
/>

codersdk/audit.go

@@ -48,6 +48,7 @@ const (
 	ResourceTypeAISeat        ResourceType = "ai_seat"
 	ResourceTypeAIProvider    ResourceType = "ai_provider"
 	ResourceTypeAIProviderKey ResourceType = "ai_provider_key"
+	ResourceTypeGroupAIBudget ResourceType = "group_ai_budget"
 	ResourceTypeChat          ResourceType = "chat"
 	ResourceTypeUserSecret    ResourceType = "user_secret"
 )
@@ -114,6 +115,8 @@ func (r ResourceType) FriendlyString() string {
 		return "ai provider"
 	case ResourceTypeAIProviderKey:
 		return "ai provider key"
+	case ResourceTypeGroupAIBudget:
+		return "group ai budget"
 	case ResourceTypeChat:
 		return "chat"
 	case ResourceTypeUserSecret: