fix: move CORS OPTIONS handling into nginx location block

Nginx does not allow add_header inside if blocks at the server level.
Moving the OPTIONS preflight response into the location / block fixes the
emerg directive error and allows nginx to start correctly.

nginx.conf

@@ -7,24 +7,21 @@ http {
         listen 8666;
         server_name localhost;
 
-        # CORS: allow any origin, including app://obsidian.md
+        # Global CORS headers for all responses
         add_header 'Access-Control-Allow-Origin' '*' always;
         add_header 'Access-Control-Allow-Methods' 'GET, POST, PUT, DELETE, OPTIONS' always;
         add_header 'Access-Control-Allow-Headers' 'DNT,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Range,Authorization' always;
         add_header 'Access-Control-Expose-Headers' 'Content-Length,Content-Range' always;
 
-        # Handle preflight OPTIONS
-        if ($request_method = 'OPTIONS') {
-            add_header 'Access-Control-Allow-Origin' '*' always;
-            add_header 'Access-Control-Allow-Methods' 'GET, POST, PUT, DELETE, OPTIONS' always;
-            add_header 'Access-Control-Allow-Headers' 'DNT,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Range,Authorization' always;
-            add_header 'Access-Control-Max-Age' 1728000;
-            add_header 'Content-Type' 'text/plain; charset=utf-8';
-            add_header 'Content-Length' 0;
-            return 204;
-        }
-
         location / {
+            # Handle preflight OPTIONS
+            if ($request_method = 'OPTIONS') {
+                add_header 'Access-Control-Max-Age' 1728000;
+                add_header 'Content-Type' 'text/plain; charset=utf-8';
+                add_header 'Content-Length' 0;
+                return 204;
+            }
+
             proxy_pass http://chroma:8000;
             proxy_http_version 1.1;
             proxy_set_header Host $host;