Fix duplicate Content-Type on client-metadata.json

Nginx sets application/json automatically for .json files via
MIME types; the explicit header caused a duplicate that broke
ATProto OAuth client metadata fetching.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nginx.conf

@@ -6,7 +6,6 @@ server {
 
     # Serve client-metadata.json with correct Content-Type
     location = /client-metadata.json {
-        add_header Content-Type application/json;
         add_header Access-Control-Allow-Origin *;
     }