fix(crawler): translate socks5h:// → socks5:// for Chromium --proxy-server

Chromium doesn't know the socks5h scheme (curl/reqwest convention)
and bails navigations with ERR_NO_SUPPORTED_PROXIES. It does, however,
send destination hostnames over SOCKS5 by default, so stripping the
`h` is a pure scheme rename — remote-DNS behaviour is preserved.

reqwest keeps the user's original CRAWLER_PROXY string (`socks5h://...`
remains valid and meaningful for it).

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

backend/src/crawler/url_utils.rs

@@ -91,6 +91,26 @@ pub fn registrable_domain(url: &str) -> Option<String> {
     Some(format!(".{}", registrable.join(".")))
 }
 
+/// Normalise a SOCKS proxy URL for Chromium's `--proxy-server=` flag.
+///
+/// reqwest accepts both `socks5://` (resolve locally) and
+/// `socks5h://` (resolve via the SOCKS server — important when the
+/// proxy is TOR and we don't want the host's resolver to see the
+/// target hostname). Chromium does **not** know the `socks5h` scheme
+/// and refuses navigations with `ERR_NO_SUPPORTED_PROXIES`. It
+/// already sends destination hostnames over SOCKS5 by default
+/// regardless, so stripping the `h` is a pure scheme rename — the
+/// remote-DNS behaviour is preserved.
+///
+/// Non-SOCKS schemes pass through unchanged.
+pub fn chromium_proxy_arg(proxy: &str) -> String {
+    if let Some(rest) = proxy.strip_prefix("socks5h://") {
+        format!("socks5://{rest}")
+    } else {
+        proxy.to_string()
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -191,4 +211,34 @@ mod tests {
             Some("[2001:db8::1]")
         );
     }
+
+    #[test]
+    fn chromium_proxy_arg_strips_socks5h_to_socks5() {
+        // Regression: passing socks5h:// to Chromium yields
+        // ERR_NO_SUPPORTED_PROXIES at navigation time.
+        assert_eq!(
+            chromium_proxy_arg("socks5h://127.0.0.1:9050"),
+            "socks5://127.0.0.1:9050"
+        );
+        assert_eq!(
+            chromium_proxy_arg("socks5h://tor:9050"),
+            "socks5://tor:9050"
+        );
+    }
+
+    #[test]
+    fn chromium_proxy_arg_passes_socks5_unchanged() {
+        assert_eq!(
+            chromium_proxy_arg("socks5://127.0.0.1:9050"),
+            "socks5://127.0.0.1:9050"
+        );
+    }
+
+    #[test]
+    fn chromium_proxy_arg_passes_non_socks_unchanged() {
+        assert_eq!(
+            chromium_proxy_arg("http://proxy.example:8080"),
+            "http://proxy.example:8080"
+        );
+    }
 }