feat(config): support large uploads with read header timeout

Disable default read and write timeouts (set to 0s) to prevent Go from
prematurely closing connections during large multi-GB uploads.

Introduce `WARPBOX_READ_HEADER_TIMEOUT` (defaulting to 15s) to protect
against slowloris-style attacks while still allowing long-running
uploads to complete. Update documentation and example configurations
accordingly.

backend/libs/httpserver/server.go

@@ -54,11 +54,12 @@ func New(cfg config.Config, logger *slog.Logger) (*http.Server, error) {
 	)
 
 	server := &http.Server{
-		Addr:         cfg.Addr,
-		Handler:      handler,
-		ReadTimeout:  cfg.ReadTimeout,
-		WriteTimeout: cfg.WriteTimeout,
-		IdleTimeout:  cfg.IdleTimeout,
+		Addr:              cfg.Addr,
+		Handler:           handler,
+		ReadHeaderTimeout: cfg.ReadHeaderTimeout,
+		ReadTimeout:       cfg.ReadTimeout,
+		WriteTimeout:      cfg.WriteTimeout,
+		IdleTimeout:       cfg.IdleTimeout,
 	}
 	server.RegisterOnShutdown(func() {
 		stopJobs()