feat(analysis): worker daemon + real dispatcher + app wiring

The background analysis worker that drains analyze_page jobs:

- analysis::daemon: a lean sibling of the crawler daemon — leases only
  KIND_ANALYZE_PAGE, skip-if-done-unless-force, lease heartbeat, panic +
  timeout isolation, ack done/failed, and a failed page_analysis row on
  terminal (dead-lettered) failure. AnalyzeDispatcher trait seam.
- RealAnalyzeDispatcher: load page → storage.get → VisionClient.analyze →
  persist_analysis (skips a deleted page; caps image bytes).
- app::build spawns the daemon (own plain reqwest client) when
  ANALYSIS_ENABLED; AppHandle/main shut it down alongside the crawler.
- repo::page::find_by_id.

Tests: dispatch+ack-done, skip-when-done, force re-dispatch, terminal
failure writes failed row, panic isolation, ignores non-analyze jobs.

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

backend/src/repo/page.rs

@@ -29,6 +29,19 @@ pub async fn create<'e, E: PgExecutor<'e>>(
     Ok(row)
 }
 
+/// Fetch a single page by id. `None` when it doesn't exist (e.g. deleted
+/// between an analysis job's enqueue and its dispatch).
+pub async fn find_by_id(pool: &PgPool, id: Uuid) -> AppResult<Option<Page>> {
+    let row = sqlx::query_as::<_, Page>(
+        "SELECT id, chapter_id, page_number, storage_key, content_type \
+           FROM pages WHERE id = $1",
+    )
+    .bind(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#"