feat: 交付官方 Docker 运行时，外观改背景图并下线自定义 CSS/JS

站点/后台分轨背景与用户列表排序一并落地；生产 CORS 改走 SITE_URL，健康检查带版本号。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/service/bg_test.go

@@ -0,0 +1,56 @@
+package service
+
+import "testing"
+
+func TestNormalizeBgURL(t *testing.T) {
+	okEmpty, ok := NormalizeBgURL("  ")
+	if !ok || okEmpty != "" {
+		t.Fatalf("empty should clear, got %q ok=%v", okEmpty, ok)
+	}
+	good := "/uploads/backgrounds/0123456789abcdef0123456789abcdef.webp"
+	got, ok := NormalizeBgURL(good)
+	if !ok || got != good {
+		t.Fatalf("want %s, got %q ok=%v", good, got, ok)
+	}
+	rejects := []string{
+		"http://evil.test/x.webp",
+		"/uploads/backgrounds/../avatars/x.webp",
+		"/uploads/images/0123456789abcdef0123456789abcdef.webp",
+		"/uploads/backgrounds/0123456789abcdef0123456789abcdef.gif",
+		`/uploads/backgrounds/0123456789abcdef0123456789abcdef.webp")`,
+		"/uploads/backgrounds/short.webp",
+		"javascript:alert(1)",
+	}
+	for _, u := range rejects {
+		if _, ok := NormalizeBgURL(u); ok {
+			t.Fatalf("should reject %q", u)
+		}
+	}
+}
+
+func TestNormalizeBgMode(t *testing.T) {
+	m, ok := NormalizeBgMode("")
+	if !ok || m != DefaultBgMode {
+		t.Fatalf("empty mode want cover, got %q ok=%v", m, ok)
+	}
+	if _, ok := NormalizeBgMode("zoom"); ok {
+		t.Fatal("unknown mode should fail")
+	}
+	for _, want := range []string{
+		BgModeCover, BgModeContain, BgModeRepeat, BgModeRepeatX, BgModeRepeatY, BgModeStretch, BgModeCenter,
+	} {
+		got, ok := NormalizeBgMode(" " + want + " ")
+		if !ok || got != want {
+			t.Fatalf("mode %s: got %q ok=%v", want, got, ok)
+		}
+	}
+}
+
+func TestNormalizeBgSurface(t *testing.T) {
+	if s, ok := NormalizeBgSurface("SITE"); !ok || s != BgSurfaceSite {
+		t.Fatalf("site: %q %v", s, ok)
+	}
+	if _, ok := NormalizeBgSurface("frontend"); ok {
+		t.Fatal("frontend is not a surface")
+	}
+}