fix(attic): fix nginx proxy cache bypass and add cache lock

Replace broken proxy_cache_bypass (was bypassing every request) with
proxy_cache_lock to coalesce concurrent requests for the same path.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

systems/x86_64-linux/attic/default.nix

@@ -76,7 +76,7 @@
           proxy_cache_valid 200 30d;
           proxy_cache_use_stale error timeout updating;
           proxy_cache_methods GET HEAD;
-          proxy_cache_bypass $request_method;
+          proxy_cache_lock on;
         '';
       };
     };