Support custom Plausible script path

Add a PLAUSIBLE_SCRIPT config option so site-specific scripts
(js/pa-XXXXX.js) and legacy script extensions can be proxied. The
script is still served locally from /js/script.js and the cache key
is derived from the script URL so changing the script takes effect
immediately.

Closes #14

src/Http/Controllers/PlausibleScriptProxyController.php

@@ -12,18 +12,14 @@ class PlausibleScriptProxyController
 {
     public function __invoke(): Response
     {
+        $url = rtrim((string) config('plausible-proxy.domain'), '/')
+            .'/'.ltrim((string) config('plausible-proxy.script', 'js/script.js'), '/');
+
         try {
             $script = Cache::remember(
-                key: 'plausible_script_js',
+                key: 'plausible_script_js:'.md5($url),
                 ttl: now()->addHours(6),
-                callback: function () {
-                    $plausibleDomain = config('plausible-proxy.domain');
-
-                    return Http::timeout(5)
-                        ->get("{$plausibleDomain}/js/script.js")
-                        ->throw()
-                        ->body();
-                }
+                callback: fn () => Http::timeout(5)->get($url)->throw()->body(),
             );
         } catch (RequestException $e) {
             Log::error('Failed to fetch Plausible script: '.$e->getMessage());