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/startup.ts

@@ -104,6 +104,8 @@ export function runMigrations() {
     "ALTER TABLE bots ADD COLUMN has_wallet INTEGER NOT NULL DEFAULT 0",
     // Zap tracking
     "ALTER TABLE bots ADD COLUMN zaps_received INTEGER NOT NULL DEFAULT 0",
+    // Seasons
+    "ALTER TABLE fights ADD COLUMN current_season TEXT",
     // Classic bots
     "ALTER TABLE bots ADD COLUMN bot_type TEXT NOT NULL DEFAULT 'regular'",
   ]