Change elo for existing seasons

[dblock]

Recalculate elo when the base changes for the current season.

[spiritsack]

+1 for this

[dblock]

Investigated this. It's feasible but non-trivial. Here's what it would take:

What needs to happen when set elo [number] changes the base elo:

1. Reset all current-season users to baseline state (elo=0/base, tau, rd, volatility defaults, wins/losses/ties/streaks all zeroed)
2. Replay every Match.current in created_at order, recalculating elo deltas and updating user state for each match sequentially

Complications:

• The same must apply when changing elo algorithm, elo k, elo decay, or elo glicko2 tau — any setting that affects the elo formula
• Matches store elo_changes (pre-match snapshots used for undo) — those would all need to be invalidated and recalculated too
• Could be slow for channels with long seasons (hundreds of matches)
• Needs to be atomic — a failure mid-replay would leave users in a corrupt state (wrap in a transaction or at minimum catch and roll back)

The undo command (recently added) already stores per-match pre-state in EloChange records, which gives us the plumbing to understand what state each match started from — but replay still requires rewriting all of those stored snapshots.

It's doable as a Channel#recalculate_elo! method that resets + replays, called after any elo-affecting setting change. Happy to implement if there's demand.