feat(kol): add SSE streaming for prompt assist

- Extract assist runtime (prompt building, response parsing, finalize)
  from the worker into kol_assist_runtime so the sync streaming path
  and the async task path share the same logic.
- Add POST /kol/manage/assist/stream as a Server-Sent Events endpoint
  emitting start/delta/completed/error events, and wire it to a
  streamAssist helper on kolManageApi that reports ApiClientError with
  the server's error envelope.
- Stream generated content live in KolPromptEditor and KolGenerateView,
  switching KolPromptEditArea to a boolean aiBusy flag and refining the
  generator page layout.
- Add KolAssistStreamEvent to shared types.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/shared-types/src/index.ts

@@ -1331,6 +1331,18 @@ export interface KolAssistTask {
   error_message?: string;
 }
 
+export interface KolAssistStreamEvent {
+  type: "start" | "delta" | "completed" | "error";
+  mode?: "generate" | "optimize";
+  status?: "generating" | "completed" | "failed";
+  delta?: string;
+  content?: string;
+  schema?: KolSchemaJson;
+  error?: string;
+  model?: string;
+  done?: boolean;
+}
+
 export interface KolDashboardOverview {
   total_subscriber_tenants: number;
   month_new_subscriptions: number;