feat(analysis): enqueue page-analysis on upload/crawl + admin backfill endpoints

Wires the analyze_page job kind into the page-create paths and adds admin
controls, all gated on a new ANALYSIS_ENABLED flag (AppState.analysis_enabled,
config::AnalysisConfig):

- Chapter upload (api::chapters) enqueues one analyze_page job per page
  after the tx commits (best-effort; failures logged, never fatal).
- Crawler content sync (persist_pages → RETURNING id) enqueues per
  persisted page when the daemon dispatcher's analysis_enabled flag is set;
  threaded through sync_chapter_content (CLI/resync pass false).
- repo::page_analysis::enqueue_all_pages: bulk backfill via INSERT..SELECT,
  skipping done pages (only_unanalyzed) and existing pending jobs.
- New admin endpoints (RequireAdmin, 503 when disabled, audited):
  POST /admin/analysis/reenqueue and POST /admin/pages/:id/analyze (force).

Tests: upload enqueues N jobs / no-op when disabled; crawler persist_pages
enqueue gate; admin reenqueue (backfill, idempotent, only-unanalyzed, 503,
non-admin 403) and force-analyze (force flag, 404).

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

backend/src/repo/page_analysis.rs

@@ -31,6 +31,34 @@ pub async fn enqueue_for_page(pool: &PgPool, page_id: Uuid, force: bool) -> AppR
     Ok(())
 }
 
+/// Bulk-enqueue `analyze_page` jobs for existing pages — the admin
+/// backfill path for content uploaded/crawled before analysis was
+/// enabled. When `only_unanalyzed` is true, pages that already have a
+/// `done` analysis row are skipped. Pages with a pending/running
+/// `analyze_page` job are always skipped so repeated calls don't pile up
+/// duplicates. Returns the number of jobs enqueued.
+pub async fn enqueue_all_pages(pool: &PgPool, only_unanalyzed: bool) -> AppResult<u64> {
+    let result = sqlx::query(
+        r#"
+        INSERT INTO crawler_jobs (payload)
+        SELECT jsonb_build_object('kind', 'analyze_page', 'page_id', p.id, 'force', false)
+          FROM pages p
+         WHERE ($1 = false OR NOT EXISTS (
+                   SELECT 1 FROM page_analysis pa
+                    WHERE pa.page_id = p.id AND pa.status = 'done'))
+           AND NOT EXISTS (
+                   SELECT 1 FROM crawler_jobs j
+                    WHERE j.payload->>'kind' = 'analyze_page'
+                      AND j.payload->>'page_id' = p.id::text
+                      AND j.state IN ('pending', 'running'))
+        "#,
+    )
+    .bind(only_unanalyzed)
+    .execute(pool)
+    .await?;
+    Ok(result.rows_affected())
+}
+
 /// Load a page's analysis row, if it has one.
 pub async fn load(pool: &PgPool, page_id: Uuid) -> AppResult<Option<PageAnalysis>> {
     let row = sqlx::query_as::<_, PageAnalysis>(