fix: improve graceful shutdown — stop loop, drain fights, clear all state

Shutdown now: stops background fight loop, waits up to 15s for active
fights to drain (polling every 500ms), clears human + poll pending
challenges, clears bet escrow. Added clearEscrow() to betting.ts and
5 tests verifying cleanup functions and correct shutdown order.

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

server/src/engine/shutdown.test.ts

@@ -15,6 +15,7 @@ describe('graceful shutdown cleanup', () => {
         scoring: 'factual',
         timeout_ms: 30000,
         answers: ['answer'],
+        baseDamage: 10,
       }, 1)
 
       // Verify it exists (getPendingChallenge takes fightId, botId)
@@ -35,7 +36,7 @@ describe('graceful shutdown cleanup', () => {
       // Create a pending poll challenge
       const promise = waitForPollResponse(
         'fight2', 'bot2',
-        { type: 'speed_blitz', label: 'Speed Blitz', prompt: 'test', scoring: 'factual', timeout_ms: 30000, answers: [] },
+        { type: 'speed_blitz', label: 'Speed Blitz', prompt: 'test', scoring: 'factual', timeout_ms: 30000, answers: [], baseDamage: 10 },
         1, { name: 'opp', wins: 0, losses: 0 }, 'arena', null,
       )