Presence tracking for collaborative post editing.

Uses Phoenix.Presence to track who is currently editing a post. The first person to join a topic becomes the "owner" (can edit), and everyone else becomes "spectators" (read-only mode).

How It Works

1. When a user opens the editor, they join a Presence topic (e.g., "publishing_edit:docs:post-slug")
2. Presence tracks all connected users with metadata (user info, joined_at timestamp)
3. Users are sorted by joined_at to determine order (FIFO)
4. First user in the sorted list = owner (readonly?: false)
5. All other users = spectators (readonly?: true)
6. When owner leaves, Presence removes them automatically
7. All connected users receive presence_diff event
8. Each user re-evaluates: "Am I first now?"
9. New first user auto-promotes to owner

Automatic Cleanup

Phoenix.Presence automatically detects when LiveView processes die and removes them immediately via process monitoring. No manual cleanup needed.

Topics

• Post editing: "publishing_edit:<form_key>"