feat(analysis): live SSE event stream for the admin dashboard

Broadcasts analysis progress so the dashboard updates live:

- analysis::events: AnalysisEvents broadcaster + AnalysisEvent
  (Enqueued / Started / Completed / Failed), carrying the
  manga/chapter/page breadcrumb.
- The worker daemon resolves each page's breadcrumb (repo::page::locate)
  and publishes Started before dispatch and Completed/Failed after.
- The admin reenqueue publishes Enqueued (scoped by manga/chapter).
- GET /v1/admin/analysis/status/stream — SSE (RequireAdmin) forwarding
  each event as a named `analysis` frame; broadcast lag emits a `lagged`
  frame. AppState carries the always-present events bus.

Tests: worker publishes started+completed (with breadcrumb) and failed;
SSE route is admin-gated (403 non-admin) and returns text/event-stream.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/repo/page.rs

@@ -42,6 +42,22 @@ pub async fn find_by_id(pool: &PgPool, id: Uuid) -> AppResult<Option<Page>> {
     Ok(row)
 }
 
+/// Resolve a page's breadcrumb `(manga_id, chapter_id, page_number)` for
+/// live event payloads. `None` if the page doesn't exist.
+pub async fn locate(
+    pool: &PgPool,
+    page_id: Uuid,
+) -> AppResult<Option<(Uuid, Uuid, i32)>> {
+    let row: Option<(Uuid, Uuid, i32)> = sqlx::query_as(
+        "SELECT c.manga_id, p.chapter_id, p.page_number \
+           FROM pages p JOIN chapters c ON c.id = p.chapter_id WHERE p.id = $1",
+    )
+    .bind(page_id)
+    .fetch_optional(pool)
+    .await?;
+    Ok(row)
+}
+
 pub async fn list_for_chapter(pool: &PgPool, chapter_id: Uuid) -> AppResult<Vec<Page>> {
     let rows = sqlx::query_as::<_, Page>(
         r#"