fix(crawler): wrap wait_for_navigation in 30s timeout (0.36.1)

A hung TLS handshake or a page that never fires load could wedge a
worker (or the cron metadata pass) indefinitely — chromiumoxide
imposes no navigation timeout of its own.

New crawler::nav::wait_for_nav caps each navigation at NAV_TIMEOUT
(30s) and returns a typed NavError so timeouts surface as transient
(retryable) errors. Wired at the three navigation sites:
- source::target::navigate (catalog/detail/pagination)
- content::sync_chapter_content (chapter reader)
- session::fetch_probe_html (session probe)

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

backend/src/crawler/content.rs

@@ -111,7 +111,9 @@ pub async fn sync_chapter_content(
         .new_page(source_url)
         .await
         .with_context(|| format!("open chapter page {source_url}"))?;
-    page.wait_for_navigation().await.context("wait for chapter nav")?;
+    crate::crawler::nav::wait_for_nav(&page)
+        .await
+        .context("wait for chapter nav")?;
 
     let html = page.content().await.context("read chapter html")?;
     page.close().await.ok();