Backport (#11686)

* Ensure the MeUserSerializer correctly marks fields as read-only

* fix behaviour

* Add note to plugin docs.

* Adjust logic for PluginListTable

* Add superuser scope to PluginInstall API endpoint

* Update unit test for API endpoint

* Explicitly set PLUGINS_INSTALL_DISABLED if PLUGINS_ENABLED = False

* Check for superuser permission in installer.py

* Additional user checks

* Sanitize package name to protect against OS command injection

* fix(security): use SandboxedEnvironment for PART_NAME_FORMAT rendering

- Switch jinja2.Environment to jinja2.sandbox.SandboxedEnvironment in
  part/helpers.py to prevent SSTI via template tags in PART_NAME_FORMAT.
- Set pk=1 on the dummy Part instance in the validator to ensure
  conditional expressions like {% if part.pk %} are properly evaluated
  during validation, closing the sandbox bypass vector.

Fixes GHSA-84jh-x777-8pqq

* Disable some unit tests for backport

* Fix SSRF in remote image download

Add IP address validation to prevent Server-Side Request Forgery
when downloading images from remote URLs. The resolved IP is now
checked against private, loopback, link-local, and reserved ranges
before connecting.

Redirects are followed manually (up to 5 hops) with SSRF validation
at each step, preventing redirect-based bypass of URL format checks.

* Style fixes

* fix styles

* fix test

* Reintroduce unit tests

---------

Co-authored-by: Paul <morimori-dev@github.com>
Co-authored-by: tikket1 <chrisveres1@gmail.com>
Co-authored-by: Matthias Mair <code@mjmair.com>

docs/docs/plugins/install.md

@@ -74,6 +74,9 @@ Enter the package name into the form as shown below. You can add a path and a ve
 
 {{ image("plugin/plugin_install_txt.png", "Plugin.txt file") }}
 
+!!! info "Superuser Required"
+    Only users with superuser privileges can manage plugins via the web interface.
+
 #### Local Directory
 
 Custom plugins can be placed in the `data/plugins/` directory, where they will be automatically discovered. This can be useful for developing and testing plugins, but can prove more difficult in production (e.g. when using Docker).