fix: use exact location match for /api to prevent 301 on POST

location /api/ caused nginx to 301-redirect POST /api -> /api/
Browsers follow 301 with GET, breaking JSON-RPC. Exact match
location = /api proxies directly without redirect.

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

nginx.conf

@@ -9,11 +9,14 @@ server {
     }
 
     # Proxy API calls to the backend service
-    location /api/ {
+    # Exact match on /api — avoids the 301 redirect nginx emits for
+    # location /api/ when the client posts to bare /api (POST→GET on 301)
+    location = /api {
         proxy_pass http://backend:5000/;
         proxy_set_header Host $host;
         proxy_set_header X-Real-IP $remote_addr;
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
     }
 
     # Rewrite /{lang}/result/{id} to /{lang}/result/id/index.html