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"
/>

coderd/audit.go

@@ -552,6 +552,18 @@ func (api *API) auditLogResourceLink(ctx context.Context, alog database.GetAudit
 		// TODO(PLAT-102): point at the user secrets management page once
 		// it ships. Until then, the audit row links nowhere.
 		return ""
+	case database.ResourceTypeGroupAiBudget:
+		// The resource_id is the group's UUID; link to the group's
+		// settings page.
+		group, err := api.Database.GetGroupByID(ctx, alog.AuditLog.ResourceID)
+		if err != nil {
+			return ""
+		}
+		org, err := api.Database.GetOrganizationByID(ctx, group.OrganizationID)
+		if err != nil {
+			return ""
+		}
+		return fmt.Sprintf("/organizations/%s/groups/%s", org.Name, group.Name)
 	default:
 		return ""
 	}