feat(crawler): live status via SSE instead of polling

Replace the dashboard's 5s polling with a Server-Sent Events stream:

- StatusHandle gains a tokio `watch` version bumped on every mutation;
  GET /admin/crawler/stream subscribes and pushes a composed snapshot
  immediately on connect, then on every status change (instant, no
  lost-wakeup) plus a 5s backstop for DB queue counts / browser phase.
- Non-status signals poke the notifier so they push immediately too:
  session-expired (worker), session update / clear-expired / browser
  restart (endpoints).
- compose_status is shared by the one-shot GET and the stream; the stream
  tolerates transient DB errors with a keep-alive comment instead of
  tearing down.

Frontend: the crawler page opens an EventSource on mount and closes it on
destroy, so the subscription is scoped to the active page (no global
subscription). A one-shot fetch still paints initial state / serves as a
fallback if SSE is blocked; a live/reconnecting indicator reflects the
connection. The existing reverse proxy already streams SSE (its abort
timer is cleared once response headers arrive), so no proxy change needed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/api/admin.test.ts

@@ -18,6 +18,7 @@ import {
     resyncManga,
     resyncChapter,
     getCrawlerStatus,
+    crawlerStatusStreamUrl,
     runCrawlerPass,
     restartCrawlerBrowser,
     updateCrawlerSession,
@@ -356,6 +357,10 @@ describe('admin crawler api client', () => {
         queue: { pending: 2, running: 1, dead: 4 }
     };
 
+    it('crawlerStatusStreamUrl points at the SSE endpoint under the API base', () => {
+        expect(crawlerStatusStreamUrl()).toMatch(/\/v1\/admin\/crawler\/stream$/);
+    });
+
     it('getCrawlerStatus GETs /v1/admin/crawler', async () => {
         fetchSpy.mockResolvedValueOnce(ok(statusFixture));
         const s = await getCrawlerStatus();