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/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.124.10"
+version = "0.124.11"
 edition = "2021"
 default-run = "mangalord"