Ecto schema for the parties table.

A party is a pre-lobby grouping mechanism. Players form a party before creating or joining a lobby together. The party leader controls when the party enters a lobby, and all members join atomically.

Rules:

• A party has a leader (creator) and members.
• Members join via invite (notification-based).
• The leader sets max_size (capacity).
• If the leader leaves, the party is disbanded (deleted).
• When the leader creates or joins a lobby, all party members join that lobby atomically (the lobby must have enough space).
• A user can be in both a party and a lobby simultaneously.
• A user can only be in one party at a time.