feat: add season management to the admin panel

[adrunkhuman]

Goal

Expose the minimum season management needed in the admin surface.

Depends on

• season schema and active-season migration
• season-scoped fixture/standings behavior

Scope

Add admin-panel support for the basic lifecycle:

• view current active season
• create/start a new season
• end/archive the current season
• switch active season only if the implementation can keep the rule simple and safe

Behavior questions to resolve during implementation

• should ending a season require all fixtures to be closed?
• should starting a new season automatically end the old active season?
• should switching active season be allowed after both seasons have fixtures?

Recommended defaults

• one active season per guild
• starting a new season ends the previous active season unless there is an open fixture
• do not allow ending a season with open fixtures unless there is a clear admin confirmation path

Non-goals

• no season scheduling engine
• no parallel competitions
• no per-season channel configuration
• no complex historical browsing panel

Acceptance criteria

• server admins can see which season is active
• server admins can create a new season without touching the database manually
• active-season changes are reflected by fixture creation and standings
• unsafe transitions fail with clear admin-facing messages
• admin panel tests cover the lifecycle paths