feat: add upload policies, daily limits, and storage quotas

- Add environment variables to configure anonymous uploads, daily upload caps, and default user storage limits.
- Update config loader to parse and validate the new settings.
- Implement backend logic to track daily usage and active storage per user.
- Update README and `.env.example` to document the new settings and admin panels.

backend/templates/pages/admin_settings.html

@@ -0,0 +1,64 @@
+{{define "admin_settings.html"}}{{template "base" .}}{{end}}
+
+{{define "content"}}
+<section class="app-shell admin-shell" aria-labelledby="admin-settings-title">
+  <aside class="app-sidebar">
+    <a class="sidebar-link" href="/admin">Overview</a>
+    <a class="sidebar-link" href="/admin/files">Files</a>
+    <a class="sidebar-link" href="/admin/users">Users</a>
+    <a class="sidebar-link is-active" href="/admin/settings">Settings</a>
+    <a class="sidebar-link" href="/app">My files</a>
+    <form class="sidebar-logout" action="/admin/logout" method="post">
+      <button class="button button-outline" type="submit">Logout</button>
+    </form>
+  </aside>
+
+  <div class="app-main">
+  <div class="admin-header">
+    <div>
+      <p class="kicker">Operator console</p>
+      <h1 id="admin-settings-title">{{.Data.PageTitle}}</h1>
+    </div>
+  </div>
+
+  <div class="card admin-table-card">
+    <div class="card-content">
+      <div class="table-header">
+        <div>
+          <h2>Upload policy</h2>
+          <p>Values are stored in megabytes. Admin users bypass these upload caps.</p>
+        </div>
+      </div>
+
+      <form class="settings-form" action="/admin/settings" method="post">
+        <label class="checkbox-field">
+          <input type="checkbox" name="anonymous_uploads_enabled" {{if .Data.Settings.AnonymousUploadsEnabled}}checked{{end}}>
+          <span>Allow anonymous uploads</span>
+        </label>
+        <label>
+          <span>Anonymous max upload MB</span>
+          <input name="anonymous_max_upload_mb" value="{{.Data.Settings.AnonymousMaxUploadMB}}" required>
+        </label>
+        <label>
+          <span>Anonymous daily upload MB per IP</span>
+          <input name="anonymous_daily_upload_mb" value="{{.Data.Settings.AnonymousDailyUploadMB}}" required>
+        </label>
+        <label>
+          <span>User daily upload MB</span>
+          <input name="user_daily_upload_mb" value="{{.Data.Settings.UserDailyUploadMB}}" required>
+        </label>
+        <label>
+          <span>Default user storage MB</span>
+          <input name="default_user_storage_mb" value="{{.Data.Settings.DefaultUserStorageMB}}" required>
+        </label>
+        <label>
+          <span>Usage retention days</span>
+          <input type="number" name="usage_retention_days" min="1" value="{{.Data.Settings.UsageRetentionDays}}" required>
+        </label>
+        <button class="button button-primary" type="submit">Save settings</button>
+      </form>
+    </div>
+  </div>
+  </div>
+</section>
+{{end}}