fix(nginx): bump /api/ proxy_read_timeout 300s → 600s for ESP-IDF builds

Cold ESP-IDF compiles (esp32, esp32-cam, etc.) can legitimately take
5–10 minutes — first time the project is built, ninja has to compile
the entire IDF + Arduino-ESP32 component graph from scratch. Nginx was
cutting the connection at 5 min, which surfaced as 'No response from
server' in the frontend even though the build was still progressing.

Worse, since the backend doesn't cancel the compile on client
disconnect, repeated user clicks pile up parallel ninja jobs that
saturate CPU and slow every concurrent build further.

10 min covers cold builds with comfortable margin. Long-term we should
make compile a job (POST → job_id, GET status) and cancel duplicates
server-side, but the timeout bump unblocks users today.

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

docker/nginx.conf

@@ -53,7 +53,7 @@ server {
         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;
-        proxy_read_timeout 300s;
+        proxy_read_timeout 600s;
         proxy_connect_timeout 75s;
     }