fix(webview): route webview:changed only to its owner in multi-user mode

webview:changed carried only {action, id} and the SSE routing hint had no
webview: branch, so every connected client received it: in multi-user mode
any user saw the ids of other users' web-tab creates, edits and deletes.
The event now carries the web tab's owner (from the stored record) and is
routed to that owner plus admins. Single-user delivery is unchanged.

src/web/webview-sse.ts

@@ -0,0 +1,6 @@
+/** @fileoverview Trusted owner routing metadata for saved-webview invalidations. */
+import type { SseRoutingHint } from './sse-stream-manager.js';
+
+export function deriveWebviewSseHint(data: unknown): SseRoutingHint {
+  return { username: (data as { owner?: string }).owner, sessionScoped: true };
+}