Implement Student Soft Delete & Filter Tabs

[americac]

!!! Blocked By: #941 !!!

Description:
Add soft delete/restore functionality with filter tabs for Students.

Tasks:

• Update Admin::StudentsController:
  • Override destroy to use discard instead of hard delete
  • Add restore action for undiscarding students
  • Implement scoped_resource to filter by discarded status (:discarded, :all)
• Update views:
  • Create custom index header with filter tabs (Active, Discarded, All)
  • Create custom collection item actions with Restore/Discard buttons
  • Add Turbo confirmation dialogs
• Add query param handling for filter switching

Files to Create/Update:

• app/views/admin/students/_index_header.html.erb
• app/views/admin/students/_collection_item_actions.html.erb

Reference:

• app/controllers/admin/students_controller.rb (lines 42-45, 47-54)
• app/views/admin/students/_index_header.html.erb (current)
• app/views/admin/students/_collection_item_actions.html.erb (current)
• app/dashboards/student_dashboard.rb (COLLECTION_FILTERS)