Add rake task to migrate CarrierWave files to ActiveStorage

[farhan-ahmed1]

This pull request adds a rake task to migrate existing database backup files from CarrierWave to ActiveStorage.

Changes:

• migrate_backups_to_activestorage.rake - Added db_backups:migrate_to_activestorage rake task
• Migrates DatabaseBackup records from CarrierWave (file) to ActiveStorage (backup_file)
• Migrates ResourceContent records from CarrierWave (sqlite_db) to ActiveStorage (sqlite_database)
• Uses memory-safe Tempfile streaming for large files
• Supports dry-run mode
• Is idempotent, so it skips already migrated records

Testing:

# Preview migration
DRY_RUN=true rake db_backups:migrate_to_activestorage

# Run migration
rake db_backups:migrate_to_activestorage

[farhan-ahmed1]

@naveed-ahmad Please test this task and review. Thank you