Add student import to new admin students page

[americac]

Closes #943

This PR implements CSV bulk import functionality for Students in the new admin interface (/admin-new/students), matching the behavior of the existing admin import system.

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Changes Made

Routes (config/routes.rb)

• Added POST /admin-new/students/import route for CSV file uploads
• Added GET /admin-new/students/template route for downloading CSV template

Controller (app/controllers/admin_v2/students_controller.rb)

• New Actions:
  • import - Handles CSV file upload, validates file presence/format, processes imports via BulkStudentImportService
  • template - Generates and downloads CSV template file
• Helper Methods:
  • redirect_with_missing_file_error - Handles missing file validation
  • redirect_with_import_results - Processes and displays import results
    - redirect_with_no_results_error - Handles empty CSV files
  • partition_results - Separates results into created/skipped/failed categories
  • build_success_messages - Builds user-friendly success messages
  • build_created_message - Formats created student messages with usernames
  • build_skipped_message - Formats skipped student messages
  • redirect_with_mixed_results - Handles partial success scenarios with both success and error flash messages

View (app/views/admin_v2/students/index.html.erb)

• Added "Template" button to download sample CSV
• Added "Import Students" button to open import modal
• Added professional Tailwind-styled modal with:
  • File upload field (.csv files only)
  • CSV requirements displayed: classroom_id, username
  • Cancel and Import buttons
  • Close button and background overlay dismiss

Tests (test/controllers/admin_v2/students_controller_test.rb)

• Added 10 comprehensive tests covering:
  • Template download functionality
  • Successful CSV imports
  • Duplicate username handling (skip existing)
  • Error reporting with line numbers
  • Missing file validation
  • Malformed CSV handling
  • Empty CSV handling
  • Mixed success/error scenarios
  • Authorization (admin-only access)

Configuration (.gitignore)

• Added .DS_Store and **/.DS_Store to prevent macOS metadata files from being tracked

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Features

✅ CSV Template Download - One-click download of properly formatted CSV template
✅ Bulk Student Import - Upload CSV files to create multiple students at once
✅ Smart Duplicate Handling - Automatically skips existing usernames
✅ Detailed Error Reporting - Shows specific line numbers and error messages for failed imports
✅ Partial Success Support - Displays both success and error messages when some students succeed and others fail
✅ CSV Format Validation - Detects and reports malformed CSV files
✅ Admin-Only Access - Import and template features restricted to admin users
✅ Professional UI - Modern Tailwind-styled modal matching admin_v2 design system

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CSV Format

classroom_id,username
1,student001
1,student002
2,student003

Required columns: classroom_id, username

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Testing

• Manual testing completed with sample CSV files
• All edge cases verified (empty files, malformed CSV, invalid data, duplicates)
• Error messages display correctly with line numbers
• Success messages show created usernames

Sample import file included: student_import_sample.csv (10 students across 2 classrooms)

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Statistics

• 5 files changed
• 299 insertions
• 0 deletions

Screenshots