Replace admin-only broken link filter with user-accessible sidebar toggle

[Copilot]

Summary of changes

• Converted hardcoded admin-only broken link filtering to a boolean sidebar facet (include_broken_links)
• Removed special privilege check from searchable.rb
• Added filter UI to Events and Materials sidebars following the pattern of include_expired and include_disabled
• Added documentation for the include_broken_links query parameter in the OpenAPI specs (public/api/definitions/tess.yml)

Motivation and context

Fixes #1176

Previously, resources with broken links were automatically hidden via a hardcoded Solr facet in searchable.rb (lines 108-113) unless the user was an admin. This made broken links invisible and undiscoverable to regular users.

The new implementation:

• Adds include_broken_links as a special facet in lib/facets.rb applicable to models with link_monitor
• Filters by the existing failing boolean field in Solr
• Defaults to hiding broken links (preserves existing behavior)
• Exposes toggle control to all users in the sidebar
• Documents the API query parameter for /events and /materials endpoints

Screenshots

(N/A - UI change follows existing toggle pattern)

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree
  to license it to the TeSS codebase under the
  BSD license.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add filter to show events and materials with broken links</issue_title>
<issue_description>Currently, events and materials with broken links are hidden using a solr facet that is always applied (unless you are an admin):

TeSS/app/models/concerns/searchable.rb

Line 111 in b36c528

without(:failing, true)

Replace this with a boolean facet in the sidebar, as we do for include_expired and include_hidden.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add filter to show events and materials with broken links #1176

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[fbacall]

@copilot can you add documentation for this filter in the OpenAPI specs: public/api/definitions/tess.yml ?

[fbacall]

@copilot can you add documentation for this filter in the OpenAPI specs: public/api/definitions/tess.yml ?

[Copilot]

@copilot can you add documentation for this filter in the OpenAPI specs: public/api/definitions/tess.yml ?

Added documentation for the include_broken_links filter to the OpenAPI specs for both /events and /materials endpoints in commit 69d742f.