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/mod.rs

@@ -20,6 +20,7 @@ pub mod daemon;
 pub mod detect;
 pub mod diff;
 pub mod jobs;
+pub mod nav;
 pub mod pipeline;
 pub mod rate_limit;
 pub mod safety;