raise upload size cap to ~1.86 GB and nginx timeouts to 1200s

Raise RequestSizeLimit/MultipartBodyLengthLimit on upload+replace-audio,
MaxUploadBytes in BatchUpload/BatchEdit, and DefaultResponseTimeoutSeconds to
1200s. Add client_max_body_size 2000m and proxy_read/send_timeout 1200s to the
nginx manager/public confs.

deploy/nginx/deepdrft-public.conf

@@ -15,5 +15,11 @@ server {
         # WebSocket support (Blazor Server SignalR circuits + WASM interop)
         proxy_set_header Upgrade    $http_upgrade;
         proxy_set_header Connection $http_connection;
+
+        # Blazor Server SignalR circuits can be long-lived. Raise timeouts above the 60 s
+        # nginx default so idle-but-active circuits (e.g. during audio streaming) are not
+        # silently dropped.
+        proxy_read_timeout 1200s;
+        proxy_send_timeout 1200s;
     }
 }