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/config/config_test.go

@@ -1,6 +1,9 @@
 package config
 
-import "testing"
+import (
+	"testing"
+	"time"
+)
 
 func TestParseMegabytes(t *testing.T) {
 	tests := map[string]int64{
@@ -49,3 +52,20 @@ func TestEnvBool(t *testing.T) {
 		t.Fatalf("envBool() did not fall back to true")
 	}
 }
+
+func TestLoadDefaultsUseLargeUploadFriendlyTimeouts(t *testing.T) {
+	t.Setenv("WARPBOX_BASE_URL", "http://example.test")
+	cfg, err := Load()
+	if err != nil {
+		t.Fatalf("Load returned error: %v", err)
+	}
+	if cfg.ReadHeaderTimeout != 15*time.Second {
+		t.Fatalf("ReadHeaderTimeout = %s, want 15s", cfg.ReadHeaderTimeout)
+	}
+	if cfg.ReadTimeout != 0 {
+		t.Fatalf("ReadTimeout = %s, want 0 for long uploads", cfg.ReadTimeout)
+	}
+	if cfg.WriteTimeout != 0 {
+		t.Fatalf("WriteTimeout = %s, want 0 for long uploads", cfg.WriteTimeout)
+	}
+}