fix: opt-in CDP re-validation of headless-browser navigations (SSRF)

The reqwest DNS resolver can't see Chromium navigations, so a scraped page
that redirects the browser to an internal target (302 -> 127.0.0.1:5432, or
a rebinding hostname) was still loaded. Add crawler::intercept: an opt-in CDP
Fetch guard that re-validates every Document navigation/redirect through the
same ensure_public_target + resolved-IP check, failing internal targets.

Gated behind CRAWLER_SSRF_INTERCEPT (default OFF): enabling Fetch is a fragile
critical-path hook and the CDP wiring is not CI-verifiable (no Chromium in
CI). When off, open_page is byte-identical to browser.new_page; when on, a
guard-install failure falls back to an unguarded navigation rather than
wedging the crawl. The pure decision logic (verdict/is_blocked) is unit-tested;
an #[ignore] smoke test covers the CDP path. Validate with a manual crawl
before enabling in production.

Bump to 0.124.11.

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

backend/src/crawler/content.rs

@@ -118,8 +118,7 @@ async fn fetch_chapter_html_once(
 ) -> anyhow::Result<String> {
     guard_nav_url(source_url)?;
     rate.wait_for(source_url).await?;
-    let page = browser
-        .new_page(source_url)
+    let page = crate::crawler::intercept::open_page(browser, source_url)
         .await
         .with_context(|| format!("open chapter page {source_url}"))?;
     // Close the tab on every exit path — a `?` on wait_for_nav / content()