fix(server): gracefully shut down HTTP proxy to drain in-flight requests

On signal, main waited on wg for StartHTTPProxy's server.Listen, which never
returns because nothing called server.Shutdown(); it timed out after 10s and
process-exited, abruptly killing in-flight GraphQL requests.

Expose the proxy Fiber app and register an http-proxy Shutdown component so
server.Listen returns after Shutdown, letting connections drain. Guard the
shared app handle with a mutex (written by the StartHTTPProxy goroutine, read
by the shutdown component). Verified with build, vet, full test and full
race suite.

main.go

@@ -612,6 +612,10 @@ func main() {
 		return nil
 	})
 
+	// Register HTTP proxy so in-flight requests drain gracefully on shutdown
+	// instead of being cut off when the process exits.
+	shutdownManager.RegisterComponent("http-proxy", shutdownHTTPProxy)
+
 	// Cache shutdown is handled internally by the cache implementation
 
 	// Start monitoring server