feat: improve graceful shutdown — stop bg loop, drain fights, clear state

Shutdown now: 1) stops background fight loop, 2) waits up to 15s for
active fights to finish, 3) cancels pending human + poll challenges,
4) clears bet escrow. Added clearEscrow() to betting.ts. Tests verify
each cleanup function and shutdown sequence ordering.

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

server/src/engine/betting.ts

@@ -165,6 +165,16 @@ export async function settleBets(
   return settlements
 }
 
+/**
+ * Clear all escrow state (used during graceful shutdown).
+ * Returns the number of fights with open bets that were cleared.
+ */
+export function clearEscrow(): number {
+  const count = escrow.size
+  escrow.clear()
+  return count
+}
+
 /**
  * Get all active bets for a fight.
  */