perf(web): use content hash for static asset cache busting

Switch from timestamp-based to MD5 content hash versioning.
Cache now only invalidates when file content actually changes.

src/server/web/templates/login.html

@@ -4,7 +4,7 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>AniWorld Manager - Login</title>
-    <link rel="stylesheet" href="/static/css/styles.css?v={{ static_v }}">
+    <link rel="stylesheet" href="/static/css/styles.css?v={{ static_version('css/styles.css') }}">
     <link href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.0.0/css/all.min.css" rel="stylesheet">
     <style>
         .login-container {