feat(storage): add S3 backend support and advanced upload limits

- Introduce S3-compatible storage backend support using minio-go.
- Add configuration options for local storage limits, box limits, and rate limiting.
- Implement storage backend selection (local vs S3) for anonymous and registered users.
- Add an `/admin/storage` management interface.
- Update documentation and environment examples with the new configuration variables.

backend/templates/pages/account.html

@@ -10,6 +10,7 @@
     </nav>
     <hr class="sidebar-sep">
     <form class="sidebar-logout" action="/logout" method="post">
+      <input type="hidden" name="csrf_token" value="{{.CSRFToken}}">
       <button class="button button-outline" type="submit">Sign out</button>
     </form>
   </aside>
@@ -33,6 +34,7 @@
             </div>
           </div>
           <form class="settings-form settings-form-narrow" action="/account/password" method="post">
+            <input type="hidden" name="csrf_token" value="{{.CSRFToken}}">
             <label><span>Current password</span><input type="password" name="current_password" autocomplete="current-password" required></label>
             <label><span>New password</span><input type="password" name="new_password" autocomplete="new-password" minlength="8" required></label>
             <button class="button button-primary" type="submit">Update password</button>