Add descriptive 403 for /server endpoint

[adamoutler]

A new 403 page describes the issue faced in #1496, which is intended operation. However, the previous standard Nginx 403 message was too generic and did not give the user any further information about how to proceed when faced with a 403 from the internal /server proxy. This 403 page should fix that problem.

Summary by CodeRabbit

• New Features
  • Implemented a custom 403 Forbidden error page for internal-only endpoints. The page features branded styling and responsive design, clearly communicates that access is restricted, explains the endpoint is for internal use only, blocks external origins, and provides guidance directing users to contact an administrator or access via the standard web interface.

[coderabbitai]

📝 Walkthrough

Walkthrough

The pull request adds a custom 403 Forbidden error page and configures nginx to serve it when clients attempt to access the /server endpoint without proper authorization. This includes a new HTML file and an nginx configuration directive.

Changes

Cohort / File(s)	Summary
Custom Error Page front/403_internal.html	New static HTML file providing a styled 403 Forbidden page with messaging that /server is for internal use only, branded footer, and responsive styling.
Nginx Configuration install/production-filesystem/services/config/nginx/netalertx.conf.template	Added error_page directive mapping HTTP 403 responses to the new /403_internal.html custom error page within the /server/ location.

Possibly related PRs

• Enable proxies on docs #1450: Relates to /server endpoint handling; this PR adds custom error page display while the other PR handles reverse-proxy exposure of the endpoint.

Poem

🐰 A hoppy forbidding,
With style to be seen!
"Access denied," we sing,
But at least it's pristine!
Internal affairs, kept secret and green. 🌿

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title directly and accurately describes the main change: adding a descriptive 403 error page for the /server endpoint.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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