Add GET /api/v1/download/linux-package

Redirects to the newest MusicPlayer release's _linux.tar.gz, which has the
icon, a menu entry and install.sh. /download/linux keeps serving the bare
.x86_64, which updating in place needs. Releases without the package get 404.

Closes #34

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/openapi_test.go

@@ -81,6 +81,7 @@ func TestOpenAPIDocumentsTheV1API(t *testing.T) {
 		"GET /api/v1/character/list",
 		"GET /api/v1/download",
 		"GET /api/v1/download/linux",
+		"GET /api/v1/download/linux-package",
 		"GET /api/v1/download/list",
 		"GET /api/v1/download/windows",
 		"GET /api/v1/health",
@@ -235,6 +236,7 @@ func TestResponsesMatchOpenAPI(t *testing.T) {
 	v.check(http.MethodGet, "/api/v1/download/list", "", http.StatusOK)
 	v.check(http.MethodGet, "/api/v1/download/windows", "", http.StatusFound)
 	v.check(http.MethodGet, "/api/v1/download/linux", "", http.StatusNotFound)
+	v.check(http.MethodGet, "/api/v1/download/linux-package", "", http.StatusNotFound)
 	v.check(http.MethodGet, "/api/v1/health", "", http.StatusOK)
 	v.check(http.MethodGet, "/api/v1/version", "", http.StatusOK)
 	v.check(http.MethodGet, "/api/v1/version/history", "", http.StatusOK)