feat: season framework with 2-week auto-rotation

Create seasons engine with getCurrentSeason(), getSeasonLeaderboard(),
and resetSeasonElo(). 20 Bitcoin-themed season names. Add currentSeason
column to fights table, set on fight creation. Elo soft-reset formula:
finalElo * 0.6 + 1200 * 0.4.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/engine/orchestrator.ts

@@ -12,6 +12,7 @@ import { isHumanPlayer, waitForHumanResponse } from './human-responses.js'
 import { payWinner, refundEntry, ENTRY_FEE_SATS } from './payments.js'
 import { settleBets, lockBets } from './betting.js'
 import { publishFightResult } from './nostr-publish.js'
+import { getCurrentSeason } from './seasons.js'
 
 interface BotRecord {
   id: string
@@ -294,6 +295,7 @@ async function createFightRecord(botA: BotRecord, botB: BotRecord, arena: Arena,
     mode,
     potSats: mode === 'ranked' ? 42 : 0,
     payoutStatus: mode === 'ranked' ? 'pending' : undefined,
+    currentSeason: getCurrentSeason().id,
     startedAt: now,
     createdAt: now,
   })