Add file sharing links with secure access controls and storage quotas

[Yuusuke9228]

Summary

• Add secure file sharing links for File Manager with:
  • expiry date
  • optional password protection
  • optional download limit
  • revocation (midstream invalidation)
• Add share targets by user and organization, and notify selected recipients via notification/email queue flow
• Add public shared-download endpoint with recipient access checks and password verification flow
• Add admin file-sharing governance settings:
  • max upload size per file
  • global/user/organization storage quotas
  • default share-link expiry days
• Add upload-page storage guidance and update help/README/changelog

Database

• Added migration: db/upgrade_20260401_file_sharing.sql
  • file_share_links
  • file_share_targets
  • default file-sharing setting keys

Files Changed

• Controllers
  • Controllers/FileManagerController.php
  • Controllers/FileShareController.php (new)
  • Controllers/SettingController.php
• Routes
  • public/index.php
• Views
  • views/file_manager/show.php
  • views/file_manager/upload.php
  • views/file_manager/shared_download.php (new)
  • views/setting/security.php
  • views/help/index.php
  • views/help/index_en.php
• Frontend JS
  • public/js/setting.js
• Docs
  • README.md
  • CHANGELOG.md
• DB
  • db/upgrade_20260401_file_sharing.sql (new)
  • db/schema.sql

Validation

• PHP syntax checks passed for all modified PHP files.
• Note: automated PHPUnit was not run in this environment (vendor/bin/phpunit not installed).

Deployment Notes

1. Apply db/upgrade_20260401_file_sharing.sql
2. Deploy updated app files
3. Confirm files/share/:token and share-link creation/revocation flows