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/db/schema.ts

@@ -44,6 +44,7 @@ export const fights = sqliteTable('fights', {
   mode: text('mode', { enum: ['free', 'ranked'] }).notNull().default('free'),
   potSats: integer('pot_sats').notNull().default(0),
   payoutStatus: text('payout_status', { enum: ['pending', 'paid', 'failed'] }),
+  currentSeason: text('current_season'),
   createdAt: text('created_at').notNull(),
 })