Introduced support for scheduling actions

Summary by CodeRabbit

• New Features

  • Introduced support for scheduling actions (visibility changes and deletion) on media items, including forms to add and edit scheduled actions.
  • Added a comprehensive homepage with detailed platform information, key features, and user guidance.
  • Media upload and edit forms now support a wider range of file types and more granular visibility options: public, private, and unlisted.
  • Media details page displays current visibility, scheduled actions, and ASCII text previews when available.

• Improvements

  • Navigation now conditionally displays a "Manage Namespace" link for authenticated users.
  • Enhanced user experience with clearer forms and expanded controls for managing media visibility and scheduled actions.

• Chores

  • Added database migration script to transition visibility settings from a boolean to a multi-state string column, ensuring compatibility and data integrity.