feat(backend): serve userscript at token-protected path

Reads the file per request from USERSCRIPT_PATH and rewrites its @version
to an mtime-derived value, so Violentmonkey always sees a higher version
after an edit regardless of what the file body claims. Also guards against
ServeMux's own path-cleaning redirect turning an empty {token} segment into
a 307 instead of the required 404.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/main.go

@@ -21,6 +21,9 @@ type Config struct {
 	Port           string
 	// WebPassword gates the browser UI. Empty disables the web routes entirely.
 	WebPassword string
+	// UserscriptPath is the file served at /u/{token}/manga-bookmark.user.js.
+	// Supplied by a bindmount so the script can be edited without a rebuild.
+	UserscriptPath string
 	// LatestPoll configures the background latest-chapter fetcher.
 	LatestPoll LatestPoll
 }
@@ -128,11 +131,12 @@ func loadLatestPoll() LatestPoll {
 
 func loadConfig() Config {
 	c := Config{
-		Token:       os.Getenv("API_TOKEN"),
-		DBPath:      envOr("DB_PATH", "/data/bookmarks.db"),
-		Port:        envOr("PORT", "8080"),
-		WebPassword: os.Getenv("WEB_PASSWORD"),
-		LatestPoll:  loadLatestPoll(),
+		Token:          os.Getenv("API_TOKEN"),
+		DBPath:         envOr("DB_PATH", "/data/bookmarks.db"),
+		Port:           envOr("PORT", "8080"),
+		WebPassword:    os.Getenv("WEB_PASSWORD"),
+		UserscriptPath: envOr("USERSCRIPT_PATH", "/userscript/manga-bookmark.user.js"),
+		LatestPoll:     loadLatestPoll(),
 	}
 	for _, o := range strings.Split(os.Getenv("ALLOWED_ORIGINS"), ",") {
 		if o = strings.TrimSpace(o); o != "" {
@@ -149,6 +153,11 @@ func newRouter(store *Store, cfg Config) http.Handler {
 	mux := http.NewServeMux()
 	mux.HandleFunc("GET /healthz", healthz)
 
+	// Outside withAuth (the updater sends no Authorization header) and outside
+	// the WEB_PASSWORD gate (the script must be installable either way). The
+	// path segment carries the token instead.
+	mux.HandleFunc("GET /u/{token}/manga-bookmark.user.js", userscriptHandler(cfg.Token, cfg.UserscriptPath))
+
 	h := &bookmarkHandler{store: store}
 	protected := http.NewServeMux()
 	protected.HandleFunc("GET /bookmarks", h.list)
@@ -170,7 +179,22 @@ func newRouter(store *Store, cfg Config) http.Handler {
 		web.register(mux)
 	}
 
-	return withCORS(cfg.AllowedOrigins, mux)
+	return withCORS(cfg.AllowedOrigins, guardEmptyUserscriptToken(mux))
+}
+
+// guardEmptyUserscriptToken heads off ServeMux's own path-cleaning redirect:
+// an empty {token} segment makes the request path "/u//manga-bookmark.user.js",
+// and ServeMux 307s that to "/u/manga-bookmark.user.js" before pattern
+// matching ever runs. The endpoint's contract is 404 for any wrong token,
+// including this one, so catch it ahead of the mux.
+func guardEmptyUserscriptToken(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if strings.HasPrefix(r.URL.Path, "/u//") {
+			http.NotFound(w, r)
+			return
+		}
+		next.ServeHTTP(w, r)
+	})
 }
 
 func main() {