feat(crawler): TorController for control-port NEWNYM signaling

Minimal client over tokio::net::TcpStream — AUTHENTICATE then
SIGNAL NEWNYM, one-shot connection. Supports cookie-file and
password auth (cookie preferred when both provided); covers the
multi-line `250-...\r\n250 OK` reply form so future torrc tweaks
won't confuse the parser.

Not yet wired into the crawler — that lands in the next commits.

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

backend/src/crawler/mod.rs

@@ -26,4 +26,5 @@ pub mod rate_limit;
 pub mod safety;
 pub mod session;
 pub mod source;
+pub mod tor;
 pub mod url_utils;